Didn't think that'd be the case, I thought of that but deemed it not likely. I've got ten types of demons and I'm failing to make seven goblin civs. Maybe there's some co-requirement or something, like not being a flier or whatever, so I'll quit arguing and give it a shot.
Two or so worlds later...
Okay yeah, you were right. Turning demons up to 50 fixed the problem with not getting more goblin civs. But the more goblin civs is giving me only marginally more necromancers than just like, two big ones, although it is noticeable.
EDIT: Oh wait, nevermind, the worlds I tested with were just of the wrong "type." For some reason, there's two types of world. In one, which I'll dub 'little spread' the civs spread to adjacent tiles around their currently owned sites and tend to harshly enforce not living in evil glacier. I don't know why, but those worlds don't make towers very well. The second type, which I'll call big spread, has the civs found sites sort of close to their capital, but just sort of places them willy nilly and eventually fills in the gaps if you let them expand that much.
These aren't the greatest examples; little spread is much more clearly defined in worlds that don't have spots that get avoided, which in my case makes them skip around with site placement a bit more. And contrary to my statements, the little spread example actually has more towers than the big spread example, but that's an outlier here.
So, I've mostly got what I want now, which is really all that can be expected in the trickier worlds to gen.