I have modded my game so that all the greek factions can upgrade all greek temples, but only start their own temples. The same applies for the romans. Tried to do the same with the eastern factions but it never worked out well. Perhaps because Parthia was included. Anyway that is no more.

After having played with that for a while I notice that I didn't get the priests for the other temples I owned, only my own. So naturally I jumped in and began to change the ancilliary file. At first I removed the factions and replaced them with greek and roman where needed. The result was rather interesting.
The first turn after this one of my seleucid family members got the Priest of Vulcan (he was in a city with a temple of Hephaestus). He already had the priest of Hephaestus... I was confused, but I quicky went out to change the file again, apparently the culture name is not good enough and all the priests are applied to the proper temples be they roman or greek (the only ones changed). I changed it back to factions, but this time I included all roman and greek faction in the list for every temple.

Given I was just lucky that a priest of Vulcan popped up the first turn, I have no idea if the mod is working as it should right now. I can just imagine the Romans running around with various greek priests.