Join Date: Dec 2010
Location: Virginia Beach, VA
After a while, most cats work things out and agree to disagree and will somewhat peacefully cohabitate. I am assuming all cats are neutered? If not, that is probably a primary part of the problem.
I bought baby gates from a thrift store and stacked them one on top of another in a spare room doorway. That way the cats can see and smell each other but not get physical. After a couple days, put the resident cats in the room and let newbie explore around for a day or so, switch back and forth day after day (give it a week or so). If, after the reintroduction period the fighting continues, is physical and causes the fur to fly, then the possibility of rehoming needs to be considered. I would ignore growling or hissing, but physical fighting may mean more drastic measures, unfortunately. Not every cat is a good mix for a family of established cats. Best wishes.
Cat owners' prayer: "Lord help me be the person my cat thinks I am"