[underscore]

I know some animals are best if not introduced directly, rather you need to separate them with something like a child gate or screen door that lets them see and smell each other without the ability to actually contact each other if they don't both want to. You have to keep in mind that your grandpas cat has gone through a big change and he's having to adapt to a new house as well as your cat so it might take a while for it to come around.

That said some animals just don't get along. My inlaws had three cats, the oldest two got along fine until the youngest one came in. She's a complete asshole to other cats and they basically had what you have, one upstairs cat, one downstairs cat, and one that roamed between both. Now they're down to just the asshole cat that I fucking hate, partly because of what a fucking twat she is and partly because her shitty behaviour indirectly lead to the death of the middle cat that had an awesome personality and we all loved. Sorry for the tangent but I never thought I could hate an animal until I met this shithead.