Except for plaster buildings, Putty-covered foam, & other ground structures, in what other stuff in modeling can I use poster paints?

Basically, you're only limited by your imagination. Let trial and error be your guide. If it looks right, then poster paints were the right choice. I use poster paints to tint my plaster before I pour it in the molds. That way you get a consistent colour throughout, and if a piece gets chipped, it does not show white underneath.