They are going to hiss and growl for a few days to a week, or longer once you introduce them face to face. Once they realize they can play together they'll be fine. Last foster I had upset everyone.. and even sweet Harry was growling while GROOMING the foster. LOL. They wanted to play but they wanted to also say "you're the stranger here in my place"

Just stay with them when they are in the same room. When you can't be there, isolate the newcomer to his own room.