The catsplosion was supposed to be self-sufficient for a few game months... that's with only one set of creatures.
It would probably depend greatly on what sort of procedurally generated demons you get (if they have attacks that can kill multiple creatures at once, you're probably screwed), and maybe it could simply require some sort of mod-generated creatures that are fire-proof or the like, with fast reproductive rates, but animal caps only apply per species - with enough different species, if you threw something like 12 or more kinds of combat-capable creatures into Hell, and they could out-reproduce the kill (and spawn) rates of demons, then you'd be able to destroy Hell by virtue of attrition alone.
The only challenge is in actually getting a population base large enough on creatures dangerous enough to actually go toe-to-toe with demons. (Unfortunately, elephants appear to take 5 years to hit adulthood, so they're probably not as good as other creatures are. Still, Giant Desert Scorpions, Giant Lions, Giant Leapords, Giant Cheetahs, Giant Tigers, the non-giant versions of all those large predatory cats, all forms of bears, Horses, Cows/Bulls, Cave Crocs, all other crocodiles and alligaters, Voracious Cave Crawlers, Muskoxen, Dogs, Wolves, ... heck, even cats and other small creatures like hoary marmots just as pure cannon fodder. You might also throw down the elephants and draltha and even things like cave dragons, even if they'll likely not see their children hit breeding age, just becuase of the slim chance the breeders stay alive and let their still-large-and-powerful children do some of the fighting for them.)
Kiss your FPS goodbye, but if you leave it running for a couple months, you might actually make demons go extinct.