I've always felt anything bigger than medium was too large. I mean seriously, has anyone here ever even come close to cleaning out a large world? That'd take weeks, maybe even months of dedicated playing. And in all the time I've spent in adventure mode since I first found out about DF 3 years ago, I've killed no more than 50 beasts in my entire career. 1000 in one world would be straight up stupid; hell, even 300 (150 after worldgen) is still ridiculous. We'd want our accomplishments in the world to actually mean something. Especially in a world where most of our attention would be given to dwarf ruins. To each his own.
We don't really need playable animalmen IMO, but if you want to compromise by getting rid of a bunch of them, that's fine. There are too many in the new version as it is
I'm not sure about you, but it's not that hard to empty the world of 1000 beasts.
Average titans/beasts per player 2-5 (I never managed to get past year 2 in my 2010 games on my machines due to crashes, so it can be more, the one game I played at my friend place that hit year 5, I managed to pittrapped a mountain titan, 2 FB, killed an ettin and had one FB wandering one of the caverns).
In DFVD, culling of titans/beasts at world gen, depending on world setting: high forest, high grassland, more good tiles, more neutral tiles tends to kill them faster as well as lower pop/wealth/export req. for attacks. Yes, they die to freaking ELVES and their pointy sticks. If I recall correctly, one of the world gen I made with 80% kill-end was at ~year 100 on one of the "peaceful" settings (I think 50 or so civ), that was with 250 or so titans and 50/100 mega/semimega caves. That's 400 total, 20% of which is 80, assuming that it's 2 a piece, that would be gone easily before 40 players' turns, and that's the conservative number. If we want 200+ history, we'd need more so that they can survive the world gen. They tend to survive better on high tundra/glacier/evil worlds since they don't easily bump into any civs - elves need non-evil forest regions (forgot about savage levels), dwarves need non-savage, non-evil mountain, humans can start from any non-mountain, non-glacier, non-evil region (could be missing a few more, they were everywhere).
tl;dr We need stupid amount of titans/beasts if we want enough to survive world gen AND enough for all the players interested.