I never do introductions, because I live in a studio apartment... so it's all just one huge room. There is the bathroom, but it's TINY (only a toilet and a sink... I have to go upstairs to use the shower, and cats are FORBIDDEN upstairs).
Paizly eventually gets used to everybody else, and while she doesn't necessarily LIKE the other cats, she tolerates them. So far I've been lucky with no fights OR catching diseases (rescued 2 feral kittens with horrible URIs, but neither of my cats got so much as a sniffle!)