I had Tex for about 4 years before I adopted Georgia. I don't know if I would ever call Tex an 'aggressive' cat, but, he is an alpha, through and through. We are now 7 cats, and things have gone so smoothly (except with Monte, the 'kitten'. He is just sooooo bad!!! in a kitteny way). I think ALOT has to do with the way the introductions are handled. Slowly.

Good luck!!!