First, are all the cats spayed or neutered? Intact cats are more likely to be territorial and aggressive.
Also, make sure that the cats have their own litterboxes and their own dishes and separate areas to eat, so that there is less reason to get territorial. Give them their own space - keep the cats in separate rooms if need be, and let them out for supervised introductions.
I haven't tried any of these myself, as my girls get along just fine, but here's a few things you might try:
Feliway http://www.feliway.uk.com/It's a synthetic cat facial pheromone that is supposed to provide a calming and destressing effect. It helps with inappropriate urine marking, stress and aggression.
Flower essencesJust a couple of random pages:
http://www.catfaeries.com/essences.htmlhttp://www.rainbowcrystal.com/bach/floweran.htmlThey're supposed to provide a calming effect that restores emotional balance. There's difference essences for different situations.
Clicker traininghttp://www.clickertraining.com/Clicker training can be effective in resolving a wide range of undesirable behaviour and can help calm down cats that are aggressive. By marking and rewarding good behaviour the cat learns what is desirable and undesirable behaviours are reduced because they are not rewarding.
Anti-anxiety medicationIf there is a very high level of stress in the household and they're not integrating you might consider talking to your vet about getting some anti-anxiety medications for the cats. Your vet would be the best to advise you on what would be most appropriate.