I've always felt that rule changes are totally fine, so long as they aren't enforced retrospectively. Making a rule to handle a current situation is not fair when it kinda screws one team (yes, Byzantium), but making a rule to deal with something before it comes up again seems fine with me, because you can't possibly think of everything before the game starts.
That said, I don't think gifting away the last two cities is something that should be legislated against. If you don't like it, declare on the team that accepted the cities and deal with it in-game.
That said, I don't think gifting away the last two cities is something that should be legislated against. If you don't like it, declare on the team that accepted the cities and deal with it in-game.