I believe I can pull up what kind of temperate the random map is somewhere although I haven't tried yet.
Having different unique vassal civs for different map types I believe is totally doable, and would be an awesome way to show off smaller civs/tribes in those regions.
Technically anything can be edited even the match setup UI, but with the scale of the mod, in order to keep it compatible with other mods, having unique vassals spawn per map temperate type automatically would be the way I would go around doing it.
Also just a quick update on the internal version, I have added true random placement of vassals on maps now, avoiding water and clearing trees on their spawn areas.