spawnunit has no compunctions about spawning impossible units; it'll spawn a DOES_NOT_EXIST unit for you just fine AFAIK
No it won't - it has to look up the creature ID based on the name you specified, and if it can't find it it'll say "Invalid race" and bail out.
This is the thing im dealing with, im not trying to spawn things with a does_not_exist tag, im trying to spawn big turtles, or hell, anything shelled that I can breed, little water on this embark, and seemingly nothing lives in it, and no sign of aquifer for an aquifer turtle, if I put in GIANT_TORTOISE MALE, i get invalid race, because unlike most of the creatures ive seen, giant tortoise has no underscores in the name, if i put in GIANT TORTOISE MALE, its a valid race, but invalid "caste" in spite of that being the species name and caste listed in the raws, at least as I understand them.
Thought, can it possibly not tell where the name of the creature ends and the caste assignment begins? So the lack of underscores makes it think im trying to spawn a Giant with the caste of Tortoise, and thus possibly adding an underscore to the creature name itself in the raws might make them spawnable? Since GIANT is its own creature, referenced as such in the files, lacking an underscore in the name of certain creatures might result in it thinking you are trying to spawn a Giant rather than a tortoise, ive seen several other animals with the same, im going to guess typo? That just feels like a remarkably stupid solution.
Edit: It was a stupid solution because I think im wrong now, ive just encountered a Honey Badger Man, and those are HONEY BADGER MAN in the files, no underscores or anything, so if I was right it would have mistaken that for trying to spawn a Honey with the caste of Badger, with an extra argument Man added on the end. Im not a programmer though, so I dont know how any of this works.