I went through this problem just 2 weeks ago - although it wasn't quite as serious as your situation. I brought home a 4-month old stray female kitten that showed up at the school where I worked - I have two 2-year-old cats (male and female.) The problem was with the kitten - she would growl and hiss every time she saw Trixie and Bo. Apparently she had had problems with other cats when she was on her own. My two cats were, for the most part, just curious about the newcomer but would also growl when provoked.

During the first several days of introduction, Jazz (the kitten) was kept in a pet carrier while "out in public" and not in her separate room. Of course, there was a lot of growling going on but she couldn't actually get to Trixie and Bo or vice versa so I didn't have to worry about physical attacks. Then, for several evenings, I would allow Jazz out on her own and everytime she growled, she was sprayed with the water bottle. She cut down on the growling quickly because she didn't like the water, plus she found out that Bo and Trixie were not going to pick on her. Since she wasn't growling, the other cats didn't respond with growls either and life has calmed down considerably around my house.

Everyone is now happy again - well, Trixie and Bo both probably wish that annoying little bundle of energy would just go away and stop attacking their tails, but everything seems to be working out. Good luck with your growing family!!