i get what you mean but it all seems obvious to me.
these 'lesser civs' are clearly lesser and unable to match you birdman for dorf. it's also unlikely they would ever become good at any craft and have the weakness of no strange moods (something that is so dwarf specific it isn't funny) so their only possible jobs are ones that they cant screw up like smelting, wood cutting and so on.
so you have 3 integration options
equals - clearly a lie but obviously the most moral choice, some dorfs may complain that about sharing with the fishpeople.
2nd class - not exactly moral but better then the dirty savages could hope to get on their own.
slaves - evil as evil can evil on an evil tuesday and requiring milta to make sure they don't run and keep working.
Not that we have a giant chip on our shoulders, or anything
Anyway, seriously, if we are going to have something where we can intigrate some other society into our own dwarven society, then there does have to be some kind of backlash against doing it poorly. If slaves are indifferentiable from every one of your dwarves except your nobles, then the word "slave" sort of loses its meaning.
If we're going to have something like slavery (and even if dwarves are ethically opposed to it in vanilla mode, goblins sure aren't), then we need to have mechanics that actually involve slaves wanting to escape and the slavers having to keep them in, as a bare minimum. Slave revolts would have to be a concern, as well. There would also have to be some appreciable difference between a slave and a regular worker (typically, that whole "you didn't pay them anything" bit) that would make slavery actually make sense, which means it would require something like the economy or slavery would just be a pointless burden on your civ.
Lex actually has a point on that, as well. Even if you somehow manage to enfranchise fishmen without doing something like the kidnapping babies to raise them "European" trick, and generally manage to go about it in the "ethical" manner, there's still the matter of social
I know I've already made this a pretty controversial topic, and this is probably only going to throw gasoline on the fire, but... what about racist dwarves?
What if you set up a fortress adjacent to an elven retreat, and goblins decimate their retreat, and the survivors seek refuge in your fortress. For some odd reason, you, as a player, don't slaughter the survivors just out of principle, and let them into your fort to work as part of your own civ. Alternately, some Fishpeople start living with your dwarves as trade gets more and more of their people coming to your fortress as some sort of migrant worker, gaining skills in some trade, and either going back to their tribe with their training, or bringing their families in to live in the fortress with them. Some of the dwarves grumble.
"Why do the primitives and refugees get to enjoy a status as good as the ones they have? Why does a fish-thing get to become a master gemsetter, and take all his training back to some fish tribe, while I'm stuck hauling, with barely enough money to buy my bread?"
If we're having assymetrically powerful races in the same cultures, then what about Race Relations as a topic?