1) Always play with the full identity deck and use the entire deck when assigning secret identities to the human players. This makes it possible that all players start as parasites. But more importantly this makes it harder for players to count cards, which is super annoying and super easy to do when playing by the official rules.
We've been playing with this rule for a few months now and haven't had any problems. However, we did realize that the reason this is not the official rule is that the parasites could in theory just agree at the beginning of the game to go to the dinner table if they have a majority.
However, this has never happened, not even once. Why? Suggesting to do this is risky because you are outing yourself as a parasite. If there isn't a Parasite majority then you will get shot. Keep in mind, this could happen at any point during the game. Maybe another possibility is that my group is selfish and competitive; everyone wants to win by themselves.