What on earth for? All that would happen would that the original race/cultural gets constantly attacked and driven out culture. All the shows would be the new races, the old race called the most dangerous threat. And lots and lots of crime from new races.
Sounds like you want suffering.

As long as you are not a total xenophobe I could see why you would want a diverse fort. The nearest thing I can think of is this mod that allows you to embark with animal peoples who are part of your Civ. I used it to embark with a bunch of Ratmen and cows to make a giant castle out of cheese.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2920983785

The biggest issue I could see is body size? It might require quite a bit of micromanaging to make sure everyone has access to clothing that will fit them - I had previously been under the impression that it wasn't possible at all (probably because I wasn't playing for a number of years) but DF Wiki suggests it is: https://dwarffortresswiki.org/index.php/Clothing#Size

You CAN have a multi-species fortress with a little bit of work in vanilla, by creating taverns/libraries/guild halls/etc. open to visitors and accepting residents of diverse species. I've had elves and humans join my dwarven forts unmodded, and after some time (I think a few years?) they'll request full citizenship. This DOES NOT work with Monster Hunters, mind, and I don't know if it works with people asking to join for "Soldiering," but people who want to join to entertain or study will eventually want full citizenship.

But that DOES still leave you with your migration waves and initial embark monocultural. And will make it a lot more likely that your artifacts and other valuables will be stolen and/or you will have to spend inordinate amounts of time on the Justice tab scheduling interviews to discover nefarious plots.

DF Wiki suggests considering creating a faux multicultural entity by using castes to get around single-creature-type restrictions on entities, but I don't think it's possible to keep reproduction sane in that case? I could be wrong. https://dwarffortresswiki.org/index.php/Modding#Modding_civilizations_.28entities.29
(Also, full disclosure, I've never had naturalized migrants marry and have kids in my forts, but I don't know if this is because they CAN'T, or because they're often married already when they come visiting (but you could easily deny petitions from anyone with a spouse if that's the issue), or because I haven't run a fort with 2+ eligible members of the same species present for long enough for the sweet music to happen.

So, uh... a way to have a functioning multicultural fortress would be neat, but might be outside of what (Workshop) mods can do, at least for now. I suspect the single-creature-per-fortress thing mentioned in the wiki modding article is probably hard coded.

Originally posted by quoting_mungo:

The biggest issue I could see is body size? It might require quite a bit of micromanaging to make sure everyone has access to clothing that will fit them - I had previously been under the impression that it wasn't possible at all (probably because I wasn't playing for a number of years) but DF Wiki suggests it is: https://dwarffortresswiki.org/index.php/Clothing#Size

You CAN have a multi-species fortress with a little bit of work in vanilla, by creating taverns/libraries/guild halls/etc. open to visitors and accepting residents of diverse species. I've had elves and humans join my dwarven forts unmodded, and after some time (I think a few years?) they'll request full citizenship. This DOES NOT work with Monster Hunters, mind, and I don't know if it works with people asking to join for "Soldiering," but people who want to join to entertain or study will eventually want full citizenship.

But that DOES still leave you with your migration waves and initial embark monocultural. And will make it a lot more likely that your artifacts and other valuables will be stolen and/or you will have to spend inordinate amounts of time on the Justice tab scheduling interviews to discover nefarious plots.

DF Wiki suggests considering creating a faux multicultural entity by using castes to get around single-creature-type restrictions on entities, but I don't think it's possible to keep reproduction sane in that case? I could be wrong. https://dwarffortresswiki.org/index.php/Modding#Modding_civilizations_.28entities.29
(Also, full disclosure, I've never had naturalized migrants marry and have kids in my forts, but I don't know if this is because they CAN'T, or because they're often married already when they come visiting (but you could easily deny petitions from anyone with a spouse if that's the issue), or because I haven't run a fort with 2+ eligible members of the same species present for long enough for the sweet music to happen.

So, uh... a way to have a functioning multicultural fortress would be neat, but might be outside of what (Workshop) mods can do, at least for now. I suspect the single-creature-per-fortress thing mentioned in the wiki modding article is probably hard coded.

Don't forget that all the guests guzzle down your booze so it may not be worth it just for diversity