One of the things I had to do when I had cats (had them for years) was introduce them gradually. Whenever we got a new kitten I was told to isolate the new cat behind a closed door. We would go in to play with him frequently. The old cat would sniff him under the door and would pick up his scent on our clothes. Then when we let them together (usually two days) they would warily approach each other but would not fight. They always settled in a couple of days.

Billye