@Mantis:
I think a good rule is that if the adult cat is there first, they can tolerate each other but never quite hang out and play.QUOTE]
Doesn't always work that way.
When Jazzy moved in we had one adult cat {12 at that time} and one kitten {7 mos at that time}. Jazzy went after them from the start. Not just chasing, but entering what I call The Zone, where she seems to hear and see nothing else when she is on the quest to get the cats. There is NO toleration.
We had the adult cat {and another one that has since died} when Gypsy moved in, and Gypsy learned quickly not to chase the cats, and they actually became very close. They'd sleep together, groom one another, etc. Our current younger cat moved in and w/in a week was sleeping with Gypsy also.
When Jazzy moved in, both our cats were curious and willing to give it a try. But from day one, it was a no-go w/Jazz. For about a year and a half the younger one would come downstairs when Jazzy was sleeping, but she doesn't even do that anymore. Both cats just inhabit the upstairs. We had another dog w/Gypsy for years, and she also loved cats. My niece has brought her dogs over and they've been able to learn to leave the cats alone.
My poor cats have never experienced a dog like Jazzy.
Jazz knows they're upstairs and sometimes suddenly jumps up and runs to the stairs and just stares and paces, even though the cats aren't even visible to her. I don't know what we'd do if our house wasn't large.