Inbred cave-billies are already in the vanilla game, in the form of Troglodites. Though it does sound like a good idea, especially if multi-race forts are going to be possible before Goblins are done. I like the idea of Trogs being the default second-class citizen in Goblin forts, with other races filling in particular niches as the goblins want their talents and/or have enough surplus immigrants to risk the rebellions. A "successful" goblin fort would be one where goblins do none of the work and reap all of the benefits, their tyranny keeping itself suspended on fanatical slave-labor.
If it's possible to change the effect of a job based on the species of a creature, I'm thinking it could go like this:
Dwarves for better smithing, faster mining (higher chance of pissing off the dwarf while handing them a weapon.)
Elves for better farming, better ranged combat (higher chance of pissing off the elf while handing them a weapon.)
Humans for more profitable black market trades, better melee combat (Same as above, yada yada.)
Orcs for better leather/bone working, supreme combat skills (Yada yada.)
Gnomes as better mechanics, animal trainers (has a chance to make animal opposed to life instead of training them.)
Succubi as better "entertainment" (Social skills boost, so succubi wandering around improve the general mood of the fortress by making friends.), better smelting (Chance to start a massive fire and/or produce a blob of magma while working.)
Warlocks for better alchemy/chemistry, raising zombies from corpses at a workshop (has a chance to turn opposed to life and gain permanent raising ability like a normal necromancer with each use of workshop.)
And then Trogs for everything else, with a much slower skill-gain rate and minus the chance for rebellion due to their low-intelligence making them more susceptible to indoctrination.