From my little experience, startpos from different mods may work only if they are not dependant on specific packs. To be clearer, if you only modify the level of the buildings or the playable factions, it will work. If you have cities with extra slots or buildings, units recruited in a specific province it will result in ctd.
These being said, I have noticed that MFR startpos are created based on an Italian version of the game. I don't know if there is a problem but the only versions of startpos modified by me that work with Itan's mod are those based on his startpos. That is why I prefer asking nicely for a extra feature because I have seen he is pretty receptive.