I, too, forget how long you have had them...and I go about integrating them differently. I let there be supervised contact once the new cat has been vet checked, and had a chance to settle into a new room...24 hours is about the average. Then I go for supervised visits...and I speak harshly to my cats if I see any signs of aggression. I know people think this isn't appropriate, but I truly think my cats "know better".

Here, I don't know if you have gone too long without interaction..and everyone is getting too protective of their turf.

Have you thought about a large dog crate...and covering up alot of it for privacy, but give everyone a chance to interact with safety in mind?