Introducing new cat next week.
I now have 5 cats, soon to be 6. When I bring in a new animal I isolate that animal in a room by himself or herself. I am fortunate to have the room as my kids are grown and gone. I gradually let the other cats see the new one, and gradually let them get acquainted. When I am at work I keep the doors closed. At this time I have an older cat that is new to the house, her owner had to go away to a nursing home. She has settled in but does let the other cats know that this is her room. She is in my study and does not try to leave the room. if you have the room to isolate the animal, do so. If you do not have room, keep a crate handy so you can put the new cat in there if it becomes disturbed. Even a large cat carrier can work. I had to do this one time with one particularly angry cat whose owner had died of cancer. When he became disturbed he would get into the carrier, feeling secure. Eventually they will become acquainted, maybe not friends.