I already have two indoor cats. My mom and her two lived with us for a while last year and there were definite issues between hers and my orange tabby, Rocket. So I doubt we can go that route, but he is free of Feline Leukemia and rabies, I’m going to try to get him to live in the back yard but we’ll see.
I agree with Hetty_Fauxvert, cats live longer indoors, are healthier and I’ve never really had a problem bringing an outside cat inside. You need to give them a chance to get acquainted like keeping the new one in an unused room so the others can smell him and get used to him being there. Then open the door.
Whatever you do, You’ve given this little guy a new lease on life.
Agree with Lx that whatever happens, you’ve certainly helped your kitty friend! So that’s a good thing. Also agree with Lx that the proper way to integrate a new cat is to keep them in a separate room ... really, the longer the better. Even a week is good, if it’s not too much trouble. By the time the week is up, Cat #1 has gotten bored with being indignant, and will probably give the newcomer only the occasional hiss when you allow them to come out. Our cat Samwise (the wonderful Manx we brought in out of the cold) was our third indoor cat, and indeed we had to referee for a couple of weeks until everything settled down. But he was just the most terrific friend ... we still miss him.
Wanted to mention also that if you really want your friend to stick around after you put him outside, you should keep him in an enclosed space outside (like a kennel) for a few days. (This was what the feral cat people told us, and they’ve had a lot of experience.) Feed him there (and of course he’ll need a litter box) and then when you let him out, he kind of sees your yard as his personal property and will stay. (That’s the theory, anyway! :)