Wandering cat has returned should I keep him in?

Before my current situation, I lived in a virtual zoo. My husband, not being a cat lover, wasn't too keen on me bringing 8 cats into the home so my daughter has the brood. But they weren't all acquired at the same time. They have learned to live together, or not. We have one who simply avoids the others because he's a bit antisocial. He hisses at the others when they get in his space and they've just learned to steer clear. But over time, they have all learned to live in the same house and even made the move to my daughters with little kerfuffle. I say let mum keep jr at a distance and they'll work out an arrangement on they're own. If 8 cats can sort it out (my brother has 17 and 5 of them are companion cats that he takes on visitations to nursing homes) I should think 2 can find a way to live together. As for the sneezing, I think I'd have a vet look at that. No telling what he was exposed to out in the wide world and better to be safe than sorry.