We have never had cats that have been welcoming to other cats and there have always been a few days of hissing, sometimes the old ones keeping away, etc but things have always settled down after a few days.

Personally, I prefer not to try to keep them separate but to let them see one another straight off. The times we have tried the other approach, the old residents have seemed to know anyway - it just seems to me to produce suspicion and unsettled cats and you still have the hissing when they meet.

Long term, I agree with everyone here - it will be good for the cat to have a friend.

Jon