You can watch to see how bad the fighting is. My four boys squabble a bit and throw in the odd scream but they have never had anymore then a tiny nip. I handle them all daily and check them for wounds so I can spot any potential problems. Sometimes you just have to let them sort it out for themselves. After a squabble they always settle down together.

You could try visiting 'Fancy Rats ' forum where you will find extensive advice.