Furbum - those are excellent suggestions! Taking time out to play with each of them (alone in a room) really helps them feel better.
Also, having 3 dimensions is so important. Height is often a position of status, and having the ability to go up in their territory can really help reduce tension.
We brought in a sixth kitty - Ming Loy. She's handicapped, can't jump, and now 1 1/2 years old has developed enough co-ordination to climb a bit. She's fast-moving on the ground and loves chasing the other cats. It would drive them insane if they couldn't swing up six feet onto the top of a cat tree in a flash!
Yes - put another litterbox in your bedroom. If that's where Squitchy is and she isn't using the box and won't go out to use another box, I'd try adding a second to the room.
Confining them together in a small space is the opposite of what should be done. Confining the new kitty by herself to a small space (but large enough for a litterbox, food, water and room to play) would have been the way to go when you first brought her home. That gives her time to get used to a new home and the people in it. And it gives her time to get used to the new smells of other pets - and it gives the other pets time to get used to her smell.
But that not having been done, unless there are serious injuries, time will work out their relationships. The most important thing is for you to spend time with each of them individually - especially Squitchy.
Also, Susan is right. Yelling at them won't help - it'll add to their tension. If you've tried other methods of behavior modification (like squirting water at the fighting kitties), this won't help either.
Cats are territorial animals, and they also have a hierarchy. The addition of a new cat has upset the applecart, and until they get it all worked out, it seems there will be some fur flying.
Feliway may help. It did for us. You can either purchase the plug-ins, or you can purchase spray bottles (you have to reapply the spray every three days or so. In our experience it doesn't stain wood, walls, or material). You should be able to find either at a pet store.
You can also consider purchasing Bach's Flower remedy for multi-cat household. This is available at www.catfaeries.com
. Put a few drops in the water, or touch a drop behind each kitty's ears.
We had five cats living in an RV. The addition of each one always came with apprehension and fighting somewhere. We found creating multi-level space and LOTS of extra play time to be the best cures for getting everyone settled again.
It's also been our experience that male cats are FAR more social than the females. The boys generally take to new kitties pretty quickly, and the girls hold out the longest. Spooky takes at least six weeks before she'll stop being aggressive to a new kitty. With the introduction of Ming Loy it took almost three months.