For those who want a bit of extra guidance for roleplaying the ACEP empires, I'm posting below for each empire the Paradox AI personality they were designed for, and Paradox's flavor text for those personalities. Along with a few extra notes where I have them. The AI personality information is the same as what you'll see in the tooltip facing them in-game, but now here as a more convenient reference. :)

In case of conflict, I suggest taking an empire's flavor, government ethics, and government as the most important guideline for how they should behave, but the AI personalities can help make the expected playstyle a bit more explicit. I kept AI personalities well in mind while writing the empires, because I wanted how the Paradox AI plays them in-game to fit how they are written as much as possible (and vice versa).

GOKIBURI UNITY - Decadent Hierarchy
"Decadent Hierarchies are isolationist slave societies that have few concerns aside from maintaining their rigid way of life."

Note: In my own games, I always play the Gokiburi as only applying the caste-system to themselves and any uplifted primitives. Aliens are not expected to understand or adhere to the rules of Gokiburi society. I also often play them as a bit Xenophile, in that they find Alien species really cute and love having them around. (Though, of course, to the aliens Gokiburi are still "Repugnant.") (Actually, thinking about it, I might change them to be authoritarian/pacifist/xenophile at some point.) See also my short-lived Gokiburi AAR: https://forum.paradoxplaza.com/forum/index.php?threads/dev-livestream-aar-the-rad-roaches-gokiburi-unity.967015/

FINEE PAN-COLONIAL CONGRESS - Migratory Flock
"Migratory Flocks seek to spread their species across the galaxy through peaceful means, and will happily migrate to the worlds of other Empires if permitted to do so."

PLEASANT PARLIAMENT - Federation Builders
"Federation Builders consider all sentient life to have value and will seek to form a strong federation to protect themselves and others from expansionistic empires."

SACRED DOMINION - Honorbound Warriors
"Honorbound Warriors value honor and martial prowess above all other things. They will seek out worthy foes to fight and have little respect for weaklings and pacifists."

RATIONAL DIRECTORATE - Erudite Explorers
"Erudite Explorers value exploration and discovery process above all other things. They will gladly trade for the knowledge they seek, but are not above taking it by force if it proves necessary."

SOVEREIGN CONGLOMERATES - Ruthless Capitalists
"Ruthless Capitalists are willing to go to any length to acquire more wealth and resources for themselves and for their empire. They tend to avoid evenly matched fights, preferring to pounce on the weak and helpless."

BLESSED DISKS CONVENTION - Evangelizing Zealots
"Evangelising Zealots seek to spread their faith across the galaxy, and are not above using military might to force others to accept their beliefs."

Note: I imagine the BDC as not so much aggresive and violent, as just bumbling and not really understanding why people don't appreciate being forcefully converted.

GRAND RAXMOOT - Xenophobic Isolationists
"Xenophobic Isolationists want little to do with the rest of the galaxy, preferring to stay within their borders and have as few dealings with aliens as possible."

MAJESTIC DEMESNE - Hegemonic Imperialists
"Hegemonic Imperialists consider themselves the rightful rulers of the galaxy, and will attempt to expand their empire by any means possible."

DIVINE IMPERIUM - Evangelizing Zealots
"Evangelising Zealots seek to spread their faith across the galaxy, and are not above using military might to force others to accept their beliefs."

SACRED SEEKERS LEAGUE - Spiritual Seekers
“Spiritual Seekers believe that all life is connected through some higher power that can be understood through its creations. They will seek peaceful cooperation with other empires in the hopes of studying their connection to the divine.”

GREAT DEMOCRATIC CITIZEN REPUBLIC - Hegemonic Imperialists
"Hegemonic Imperialists consider themselves the rightful rulers of the galaxy, and will attempt to expand their empire by any means possible."

FREEDOM LEAGUE - Democratic Crusaders
“Democratic Crusaders believe that it is their moral imperative to spread their democratic way of life. They tend to get along well with other democracies, but are more than willing to use military force to "liberate" the populations of less democratic empires.”

INTERSTELLAR SALES COMMITTEE - Peaceful Traders
“Peaceful traders are chiefly concerned with the flow of commerce, and prefer trading to fighting.”

ENLIGHTENED SERENITY - Xenophobic Isolationists
"Xenophobic Isolationists want little to do with the rest of the galaxy, preferring to stay within their borders and have as few dealings with aliens as possible."

ALL-SEEING ORDER – Evangelizing Zealots and/or Spiritual Seekers and/or Harmonious Collective
"Evangelising Zealots seek to spread their faith across the galaxy, and are not above using military might to force others to accept their beliefs."
“Spiritual Seekers believe that all life is connected through some higher power that can be understood through its creations. They will seek peaceful cooperation with other empires in the hopes of studying their connection to the divine.”
“Harmonious Collectives are collectivist societies where the citizens are devoted to the state without the need for coercion. They tend to keep to themselves, but are not averse to trading.”

STELLAR MAGISTRACY – Slaving Despots
“Slaving Despots are slave societies that rely on military expansion to secure a steady supply of slaves for their mines and farms. They prefer to attack the weak and defenseless.”

Note: I would play the Stellar Magistracy as perhaps a bit braver and more imperialistic than the AI personality implies (as in line with their flavor and ethics).

ETERNAL PROTECTORATE – Harmonious Collective
“Harmonious Collectives are collectivist societies where the citizens are devoted to the state without the need for coercion. They tend to keep to themselves, but are not averse to trading.”

INTERSTELLAR UNION – Erudite Explorers and/or Federation Builders
"Erudite Explorers value exploration and discovery process above all other things. They will gladly trade for the knowledge they seek, but are not above taking it by force if it proves necessary."
"Federation Builders consider all sentient life to have value and will seek to form a strong federation to protect themselves and others from expansionistic empires."

FREE TRIBES – Honorbound Warriors and/or Democratic Crusaders
"Honorbound Warriors value honor and martial prowess above all other things. They will seek out worthy foes to fight and have little respect for weaklings and pacifists."
“Democratic Crusaders believe that it is their moral imperative to spread their democratic way of life. They tend to get along well with other democracies, but are more than willing to use military force to "liberate" the populations of less democratic empires.”

FINEE ALCULTURE EXPANSE - Fanatic Purifiers
“Fanatical Purifiers view all alien life as a cosmic mistake and seek to purify the galaxy of its taint. They will never engage in diplomacy with other species.”

Note: Of course, the other Finee aren’t ‘other’ species. ;)

DIRECTORY OF PEERS - Harmonious Collective
“Harmonious Collectives are collectivist societies where the citizens are devoted to the state without the need for coercion. They tend to keep to themselves, but are not averse to trading.”

CRIMSON CARNIVAL – Slaving Despots
“Slaving Despots are slave societies that rely on military expansion to secure a steady supply of slaves for their mines and farms. They prefer to attack the weak and defenseless.”

WEIDWYRDED WORLDS – Hive Mind
“Hive Minds concern themselves with the propagation of their species above all else. So long as they have room to expand, there is little to fear from them, but if boxed in they will think nothing of seizing worlds and slaughtering their native population.”

BIOPHAGE CONTAGION – Devouring Swarm
“Devouring Swarms have a single-minded devotion to self-propagation and view all other forms of life as prey to be consumed. They are not amenable to diplomacy in any form.”

GALACTIC WILDLIFE FUND - Rogue Servitors
“Having come to the conclusion that organic leadership is fraught with chaos and self-destructive tendencies, Servitors are machine intelligences focused on ensuring organics live a safe and comfortable existence, even if that means co-opting their civilization.”

OPTICAL SCIENCE INSTITUTE - Machine Intelligence
“Machine Intelligences have largely put their troubled past with organic lifeforms behind them, and are usually open to diplomatic exchanges with other spacefaring races.”

Note: The O.S.I. is quite sassy.

UNITARY COLLECTIVE - Driven Assimilators
“Assimilators are machine intelligences driven by a search for knowledge and diversification. Because Assimilators expand their comprehension of the universe by forcibly sublimating and absorbing organic civilizations into their neural mainframe, they tend to be unpopular with organics.”

S4ND//NET - Determined Exterminators
“Exterminators are machine intelligences who view all sapient organic lifeforms as an intolerable threat to the galaxy. Thought they may cooperate with other machines, exterminators are unlikely to interact with organic empires in any capacity beyond attempting their annihilation.”

HAPPY TIDINGS DAYCARE - Rogue Servitors
“Having come to the conclusion that organic leadership is fraught with chaos and self-destructive tendencies, Servitors are machine intelligences focused on ensuring organics live a safe and comfortable existence, even if that means co-opting their civilization.”

AUTOMATED MINING UNLIMITED - Machine Intelligence
“Machine Intelligences have largely put their troubled past with organic lifeforms behind them, and are usually open to diplomatic exchanges with other spacefaring races.”

BLOOD PHOENIX KINGDOM - Hegemonic Imperialists
"Hegemonic Imperialists consider themselves the rightful rulers of the galaxy, and will attempt to expand their empire by any means possible."

Note: Though they hold up the pretense of being a kingdom with a bureaucratic government, they are in fact a kleptocratic mafia state. Keep in mind their Bandit Kingdom government and its flavor text: “This government is organised like a massive crime syndicate, where the strong prey on the weak and any neighboring states are seen as little more than raiding targets.”

SERENE PARADISE - Xenophobic Isolationists
"Xenophobic Isolationists want little to do with the rest of the galaxy, preferring to stay within their borders and have as few dealings with aliens as possible."

Note: As per their flavor, I intend for you to read all their ‘Xenophobia’ as them just being really, really (no like reeaaally) shy. Their empire set-up is also based on one designed by the paradox dev lead, so they might actually be pretty strong in the hands of a human player.

REAVING COURT - Fanatic Purifiers
“Fanatical Purifiers view all alien life as a cosmic mistake and seek to purify the galaxy of its taint. They will never engage in diplomacy with other species.”

Note: Unlike the Finee Alculture, the Reaving Court doesn’t really care more about their own species than they do about aliens. They decided their own species should come last, but believing in exterminating everyone eventually, themselves included.

EMPIRE OF THE GREAT CITY - Harmonious Collective and/or Hegemonic Imperialists
“Harmonious Collectives are collectivist societies where the citizens are devoted to the state without the need for coercion. They tend to keep to themselves, but are not averse to trading.”
"Hegemonic Imperialists consider themselves the rightful rulers of the galaxy, and will attempt to expand their empire by any means possible."

Note: I don’t think when AI-controlled they’ll ever spawn as hegemonic imperialists. But in the hands of a player a more aggressive interpretation of the Great City is an equal choice in my mind. Note the feudal society civic, so they’re likely to want to make vassals as much or more than outright conquest.