My current world has a couple of civilizations with 200+ goblins in them each. I checked through the goblin histories, and apparently entire generations of goblins have existed in the civilization since a handful were captured in a couple past wars. As others have said, different races can be assimilated when sites become "conquered" (as opposed to destroyed) by another race/civ. Once assimilated, the new race doesn't discriminate and it becomes possible for the once conquered tribe to rise in the ranks. I've seen dwarven mayors of human towns as well.
However, I don't think you can get other races as standard migrants- those might be hard coded to only be dwarves. However, their is only one king. When you get promoted to the Mountainhome, he'll come regardless of race. It's also possible to have your king/queen be a vampire. It's.... recommended you immediately seal him inside his throne room. Or drop him in lava/forgotten beast pit.
Hilariously, this used to work for animal men too. It was much more rare, but animal-men could be tamed by the elves and released into your fort. They wouldn't actually do any work, but they would hang around your dining room eating (meat only) and raising their social skills. Like children, this made them prime candidates for mayorship. I don't remember if you could select them for promotion to baron though. Either way.... Mayor Tony the Tiger(man) says: "This fort is GRRRRRRRRREAT!" XD Not sure if this could happen in generated history though, and it was sadly fixed before you could retire a fortress. Otherwise you could get a tigerman mayor then retire the fortress, saving him as a historical figure.
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