The type of fort built in multi-terrain provinces seems to be determined by the terrain type numbers of the terrain, with lowest number terrain given priority. Forest is 4, mountain is 8, thus forest/mountain provinces will ALWAYS produce forest forts. So will Forest/anything, in fact.
Similarly, Mountain/anything-but-forest will always net you mountain forts etc. For reference of terrain type numbers (as related to sites and mapmaking), see the Dom3DB or the terrain_math.txt
here