The main file effecting AI spawning is the species_classes file. The "randomized" variable enables or disables AI spawning, which should work as it is set to yes, but since the "non_randomized_portraits" section disables portrait sets from AI spawning, it doesn't. removing the section should enable it, but it's probably a good idea to retain it if you want AI empires disabled.
Limiting the portrait set to certain government and ethics is done with the "possible" section. Setting it to will disable gestalt empires, will limit them to imperial governments and will limit to imperial or dictatorial. This will effect both AI spawns and player empires, though the government can still be changed during the game, such as by government reforms. Since some of these limits aren't normally used by the game in this way, I'm not 100% sure this won't cause problems, but I couldn't find any.
A couple other notes:
I am unsure what the "spawn_enabled" variable does, don't see it in the base game files, and removed it during my tests.
Since the species_names file is used for AI spawning, you can change some of the namelists in it to HUMAN1, HUMAN2 or HUMAN3 to allow AI spawns with UNE, CM or Roman names, respectively.
I have seen a single case of an empire with a bugged name in my testing, %ADJECTIVE%, though I am unsure what caused it.
Finally, there is another bug I found, where heirs and starting AI rulers can get non ruler portraits, instead getting class based ones. For instance, a scientist heir or ruler will get a scientist portrait. I am unsure how to fix this.