INTRODUCING THE CATS
I think the biggest mistake in introducing a new cat is simply going too fast. If the cat gets afraid, you can undo so much positive reinforcement so quickly. As Karen Pryor says, the fear response goes quickly and deeply into the most basic part of the brain.
So it’s very important to try to prevent encounters that make any animal fearful. Repeatedly putting an animal in a fearful situation is actually training it for aggression. Go slowly and stop if anyone–home pet or new cat–shows more than mild aversion. If that happens, separate them and start over.
If you have both cats and dogs, start with the cats first. While you’re introducing the new space, move the home cats into the safe room so the new cat can explore at its own pace without any other distractions (one thing at a time). Depending on whether the new cat has lived with other animals or is naturally social, a couple of days of this won’t hurt. Again, return the cat to its safe room, especially at night.
Then try to introduce the home cats one at a time, starting with the most cat-friendly one. Of course, being already clicker trained can help the transition immensely by focusing on the fun of training and rewarding non-aggressive behavior. Then you can add cats at a level that feels comfortable to the new cat.
For most cats, some hissing, growling and possibly stalking by the home cat will be normal. As long as no one ends up in bloody combat, that will probably lessen over time. Getting used to the home cats singly should make it less likely that the new cat gets ganged up on. If you can make some play barriers, like draping a sheet over some chairs or making a little bridge like this to hide under, that will let the cats get used to playing with less direct confrontation. Again, try to prevent major fearful situations and return the new cat to the safe room at night.
