You "can" add other "races" to your entity by use of the caste system. See, creatures have castes, each of which are different in some ways. Normally, the only different castes in, say, the dwarf creature entry are the Male caste (which are all male) and the Female caste (which are all female and give birth to multiple children only rarely), and there are no other differences between those castes. Nothing stops you, however, from having a caste of "dwarves" that is identical in every way to an elf, in which case you'd have elf citizens showing up.
Of course, this has its own problems; you'd get "interracial" marriages, for one, since as far as the game is concerned those are the same species, and all marriages, even non-"interracial" ones, will have the capacity to produce different castes than either parent has. So, you can have two dwarves give birth to an "elf," or two "elves" give birth to a dwarf.
Also, I believe that darius's Friendship Enhancer, an external utility, can make it so that you can turn captured enemies into your friends and give them labors and such. If so, you could use it to turn humans, elves, kobolds, and goblins into productive (if, hopefully, fifth-class) citizens of your fort. You might want to look into it.
It is not currently possible to manufacture creatures in any way. You could, mind you, create a pet-creature that could be ordered from the caravans but that wouldn't give birth on its own, similar to a mule, and then call that creature a Golem and give it a golem-like body.
Mods can be combined, though depending on how the original modder did it it may be easier or harder. Things that modify the vanilla raws are harder to combine than things that just add new .txt files to the objects folder. If the modder was clever enough to make sure that the individual entries in their various .txt files are unlikely to have the same reference name as another modder it becomes even easier and less risky, though this is only rarely a problem with "professional" mods since they almost all do it.
In short, if your computer asks you if you want to overwrite something when you install a mod, it'll be harder to combine than one that doesn't.