[Edi]

Plains and farmland typically have similar populations to each other, 8k and up, over 20k for large provinces is not uncommon. Farmlands have a slightly higher pop.

Mountains have an average of 4k - 7k, large ones can be up to 15k while small ones are 2k - 3k. Forests are in the same range, or perhaps slightly upward.

Swamps have little population even when large, tops out at around 4k or 5k, and that's being generous. Likewise with waste.

These are mainly off the cuff estimates based on memory, you can run tests on the Faerun map, there are several multiple terrain provinces (Cormyr and Shaareach being the most prominent).

Terrain dependent sites can appear as long as the required terrain is present (e.g. Rockside Spring can be found in any combo with mountains).

I've no idea about if the site frequency modifiers are all cumulative or if only the biggest modifier is taken into account.

Edi