Cat Fancy magazine recommends keeping the new cat isolated when you introduce her (if you can have a friend bring her in when she's had her shots at the vet's and put her in her area, while you are paying lots of attention to your existing queen, then it will seem that the new kitty has just appeared). They stress the importance of not seeming to pay too much attention to the new cat right away. My significant other has five cats in a one bedroom apartment in Chicago. They have some spats still, but they all manage to stake some territory. It takes a few weeks sometimes. She has not had one in the current batch that has spraying problems. As you said, every case is different. The last cat my significant other got was also a well behaved obviously someone's pet, house cat. We think some elderly person had several and when they died, the family kicked them all out to the alley. We placed a couple of them (pretty and way too friendly to be outdoor kitties) and kept one.