So I was looking over the sites_and_pop log on a fresh world gen, when I noticed one of the goblins' dark fortresses was being run by a dwarf. On top of that, the actual goblin population of the fortress was around 10-ish, and there were a good number of elves, dwarves, and humans there too (the human pop was in the 50s), none of them slaves, and the fortress was a part of a goblin civ.
Curious, I decided to check out the other dark fortresses, and most of those were also heavily mixed (indeed, one "goblin" fortress was entirely composed of elves and humans!), usually with a goblin minority, and I counted a grand total of 3 demon leaders among the bunch. Looking at the history, it seems that most of the initial demons were both war-happy and unlucky. Many of them decided that being on the front lines of a battle was a grand idea, and ended up getting one-shot by some plucky guard (in one case, the demon was the only casualty in a battle of 206 versus two elves and a human, and his killer got immediately one-shot by an arrow
).
Since their leaders were killed in combat, the goblins took charge of their civs, and when they inevitably bit the big one non-goblin citizens had a shot at the throne. I'm not entirely sure where most of these non-goblins came from, but I can guess that most of them are assimilated prisoners, slaves, and children. I'm also unsure of where most of the goblins went, since they usually average about 10 per fortress, and the most is 60 living with one of the surviving demons.
One interesting side-effect is that not all goblin armies contain goblin soldiers. In fact, there's a battle where a goblin army made up of elves attacked an elven town.
The map also has one of the most bad-ass dwarven civs I've seen yet, The Cavernous Sacks. They've built a system of roads and tunnels in frozen, goblin-infested mountains, taken control of at least two dark fortressess, and one of their deities is a mountain goat.
edit: Here's a few examples of the "goblin" fortresses:
Songbalu, "Scourgeweasel", a dark fortress
Owner: The Wickedness of Authoring, goblins
Parent Civ: The Full Vice, goblins
Leader: Geshud Dikepassionate, dwarf
55 dwarves
6 goblins
1 demon
Arstrukrom, "Demonconstructs", a dark fortress
Owner: The Seducer of Midnights, goblins
Parent Civ: The Sullen Dread, goblins
Leader: Jathrur Wheelsgrowl, human
56 humans
5 goblins
13 human prisoners
Oslaaspuz, "Laudincests", a dark fortress
Owner: The Dread of Bands, goblins
Parent Civ: The Curse of Controlling, goblins
Leader: Läma Glacialweaver the League of Dens, elf
51 humans
8 elves
12 goblins
11 human prisoners
Xuspgasosta, "Malicewitch", a dark fortress
Owner: The Terror of Goals, goblins
Parent Civ: The Complex Wraith, goblins
Leader: Xuspgas Intensedungeon, elf
11 humans
31 elves