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! :)
You know what is amazing about the little weasel? Come peepee time, he went straight for the litter box, same with #2. I haven’t found anything on the floor.
How do they know?