I would love to see 2 tests done from someone who has the "mixed version OOS" problem (OOS's within the first few turns of running a mixed Vista/XP game):
1. Try to run a multiplayer vanilla BtS game with the same computers. Do you have any OOS within the same time you get it in FfH?
2. Try to run a multiplayer included BtS mod (nextWar's epic game may be a good one to try) in the same situation. Do you have any OOS's within the same time?
Assuming you dont have OOS's with vanilla BtS Im trying to figure out if there is something about FfH that is causing the operating system issues or if thats an issue with the native BtS but it only showsd up when your running a mod.
Ill get on this, ill run a 251 turn game for each scenario you mentioned with the civs on the lowest difficulty over my lan using one xp and one vista. Give me a few days to do it.