Well, it's a little late for the advice in this article, but reading it may help anyway. I'd Like You to Meet... Introducing Cats
We now have five. We had one. One week later we had two. Several months later we had three, though Munchie was a foster. Then we rescued another. Then Munchie was adopted out. Then we brought in a fourth.
The third that stayed with us, Spooky, was timid, scared (can you tell by her name, lol?) and HATED the other cats. Or so it seemed. She would hiss, growl and bat at the other cats. She would occassionally aggressively go after them. (We live in an RV so don't have any room/space to segregate cats when they come in). She cooled down after a few weeks, but still hissed and growled if any of the others got too near. It took about three months for her to be completely comfortable with all of them. Whenever we bring a new cat/kitten into the home (we've since had one more foster and adopted two more rescues), it takes her about six weeks to accept them. Now she's buddies with them - she'll sleep cuddly with them, groom others and be groomed (although with our alpha kitty it often erupts into a short small brawl after 15 or 20 minutes - either he wants her to stop and she won't, or she wants him to stop and he won't - but no one gets hurt, it's just noisy for a minute or so, lol!)
Bottom-line: cats are both social yet territorial, and it can take time for them to establish their "space" in a new home, and for the new "set" of cats to work out who's who and what's what in the "proper" pecking-order of the new hierarchy, etc.
At this point, I'd let them deal with it in their own time and in their own way. I'd only stop fighting if tails are flared AND ears are back and they're seriously fighting in a way that one of them is likely to get hurt. A cat lying on its back with a foot up in the air is in a defensive position, not a "I give" position, and be ready for potential trouble.
You can consider purchasing Feliway, either spray or plug-ins. This is a synthetic hormone that mimics the friendly scent-marker in cats cheeks. It is GREAT for helping to calm multi-cat households - it really helped us once we found out about it. I'd douse the house with it (at cat cheek level). If you live in the states, it can be purchased from many pet stores, or online at petsmart. Internationally, I don't know.
You can also try Bach's Rescue Remedy - it is very calming to cats. Here's a link to a thread about Bach's Rescue Remedy and stress: http://www.thecatsite.com/forums/sh...t=Rescue+Remedy