I spent the better part of this morning trying to get rid of the dwarf civs so I could go at it without migrants. I guess I should have just tried the pop cap->0 thing. Oh well. It was a fun modding experiment/experience for me.
To those who are interested, here's a little log of what happened:
First, I tried fiddling with the max age, setting it to 15:17.
However, this didn't seem to make any noticeable difference when checking the legends-- they still survived into the present. I realized that it wasn't sufficiently small to make the dwarves die out.
Then, I decided to make them incapable of survival so that they'd surely die out. I set the baby and child ages to around 100 and 200. This way, even if they lived reasonably long, they'd never grow up and therefore never breed. It was either that or remove the female caste to prevent breeding. Another alternative that crossed my mind later was to mod dwarves to not have heads. I'm sure that would prevent survival quite effectively.
Well, it certainly brought them to a satisfactory level of self destruction. So much, in fact, that every time I tried to generate a world, it was rejected.
After turning on the rejected world log, I found that it did so because the playable race was dead. Not only dead, but absolutely incapable of living.
"Alright then, Dwarf Fortress" I said to myself. "How about this curve ball?"
I went to the entities file and moved the playable race tag from dwarf to elf. I made sure my dwarves were set to self destruct again, and gave it another go.
At first, it looked like world gen would pull through, but it crashed about half way through and closed (longer than last time though). I tried again, but it still cashed. I guess the game threw a hissy fit because it thought I didn't want to play dwarves any more.
So. Back to the drawing board.
I decided to reset everything to default and try something else.
I went to the dwarf settings and took out the tags that let them use weapons or become smiths. I ran another world gen, but with the same effects as when I prevented breeding. It would seem that, upon the discovery that they were forbidden to work metal, they decided that life was empty and destroyed themselves in some mass suicide. I imagine it involved volcanoes.
By this time I had tried numerous attempts and uncounted worlds, and finally gave up.
So... yeah. Population cap. Probably my best bet.
Unless I can find a good island without any dwarves on it...
Good luck to anyone else trying to get this sort of thing to work