We've mixed and matched cats over the years, but we did have one failure. A tortoiseshell female that was scared of the other two (another tortoiseshell female and a black male) and the other two didn't care for her either. We compromised by having her upstairs - she didn't like going outside, either - and the others downstairs. She spent a happier year with my son in a student house in Manchester, but when she came back things were no better. So we found a new home for her when my daughter's university friends in Durham needed an "indoors" cat. A happy ending.

However, other than this one, we've had no long-term problems introducing new cats into an established cathold.