I know the new HFS or Forgotten Beasts or whatever may be based off of materials and so on, but is there a restriction on the animalmen generation?
Yes a large restriction in that there is no automatic generation. It's a misunderstanding arising from the new templates which allow you, when
creating a creature yourself to say "this is just the same as the frog entry with extra legs" and use the extra legs template to quickly and succintly create a many legged weasel, cow, sparrow and octopus, or a range of animals that all have firebreathing, or can talk, or are made out of animate grease. One of the templates he showed was a sort of anthropomorphiser kinda deal, and there was some confusion with people thinking that you could create a template, and the game would automatically apply it to some creatures.
Similarily,
there was a related belief that Forgotten Beasts would be based off custom animals, when they depend on some hardcoded details. Shame I know. But yeah, there are lots of materials for them to be made from. I'm not sure how custom materials interact with that.
On the 'animalmen' thing, can I get this clarified-- is 'animalmen' just a catch-all term that could mean ANYTHING-men or will they largely be based of off existing creatures? Like, could there be... I don't know, microclinemen?
It's not a cast-iron distinction. At one point Toady made an off-hand comment in the Dev-log about the new creatures not including many "-mans", which presumebly refers to things like microclinemen, or vomitmen (although I'm holding out for that last one). On the other hand when the "animalmen" societies are mentioned, I don't think it refers to Ironmen, and Mudmen having societies but rather to ratmen, antmen and probably things like troglodytes just to confuse the issue.