I have noticed some things about the frequency of the humanoid demons when generating a bunch of worlds. First off, the simplified worldgen seems to frequently have more demons than advanced world generations I've done. Almost every time, at the same world size, the simplified world has more demons running around. In the advanced world generation, I *think* there is a correlation between the total number of evil regions in the world and the number of worldgen demons, but I never went to the trouble of working out how that happened.
I would be eager to hear if anyone else knows what controls this.
Getting megabeasts to breed is difficult. They will only breed when: 1) there are fewer megabeasts in the world than the worldgen cap (so some beasts have to die to make space), 2) there are two megabeasts of the same species sharing a lair (no idea what controls this), and 3) you've got all the child-bearing stuff set up correctly in the raws. I can say from experience that you will have more success is your megabeast prefers polar regions, since there will be fewer human/dwarven/elven heroes wandering about trying to kill them. With my own personal mods, after 1000 years I usually end up with a single cave completely stuffed with ice dragons due to this.