Quote Originally Posted by fluffhead
well, my older cat is out and running around today! It's been a while, so this is good. The older one and new one are chasing each other around and they make little screaming noises, but they're not hurting each other at all and no hissing. I've never seen them get that close to each other yet, so: Is this a good thing? Maybe a beginning interaction?
Generally I'd let them interact without interference unless there is obvious aggression and/or loud and intense screaming and obvious distress. Hissing, snarling, etc are warning signs to move them apart (new one locked up, not the old one).

Good luck!