I think true hate and pee wars are rare. They happen, but not that often. In my experience, you keep them seperated for a few days until curiosity kills them! Then, you bring the new cat out in a carrier for awhile. Next, you let the new cat roam the house for a little bit while the other cat is locked in another room. After doing this, they generally get along pretty well because they are used to each other's scents. It took a month with my two, and only three days with my boyfriend's two. His are best buddies now and will not be seperated! They have bonded deeply and are very close. We refer to them as siblings.