Shade mostly nailed it, so I'll just add a couple bolds / underlines -
Shade wrote:3) Make sure that in the main living spaces that you have plenty of places for the cats to get up on. Cats like high places, and having a number of cat trees for them would be wonderful.
This right here is huge, moreso than one would think - and it goes beyond cat towers. Those are good, but if that's not cutting it, then really the best thing is ramps / shelves that are bolted into the wall and have cheap carpeting for claw traction. When cats get scared, they tend to go for high places - preferably something higher than your waist / chest when they're in that state.
Shade wrote:4) Watch 'My Cat from Hell' on Animal Planet-very informative and gives some nice tactics for dealing with problem felines.
This show is awesome - among other things they show you what sort of play will get the changes you want to see (or at least help get your cat closer to being there). On one episode they took a cat from full feral to coming out and accepting pats and cuddles in the course of a few weeks.
Shade wrote:Though, to me, the cats really need to just work things out. Let them fight, stop it if you feel it is getting too dangerous, but they need to establish a ranking system among themselves.
This one I'm not 100% in agreement on. The problem is that ranking systems can sometimes result in the smaller / less aggressive cat hiding in cupboards, behind book shelves and generally turning into an "indoor feral". We once had a situation with a large male picking on a small female, and that's how it went - the girl became a nervous wreck.
I really wish they'd had Cat From Hell back then...