Well, if Cloud was in the game by himself, there is no doubt in my mind that paralysis wouldn't be allowed, for obvious reasons. The fact that FF games are meant for a 3 character development type of game, there must be certain downfalls to the characters, after all, it is only fair for this type of gaming style. If Cloud was a single character featured in a game, which is the way we should be thinking about it, there is no way he could possibly be suseptible to Ness's mental prowess.