I e-mailed the following to the OP but am also posting this here for anyone browsing:
You want to start them in separate cages next to each other. Mix up the bedding between both cages and let them share a dust bath. The whole idea is to get them used to each other's scent. Do that for at least a week and then try short introductions in neutral territory (a bath tub works well). Introductions should only be a few minutes long at first, gradually increase the amount of time they spend together as long as there's no aggression. Continue introductions for a couple of weeks until you're confident that they seem to be getting along. When that happens, put them all together in a large, freshly cleaned cage (you can also let them take a dust bath right beforehand and then place that in the cage).
Small arguments and mounting can be normal-- don't separate them immediately but take that as a sign to do shorter meetings and go slowly. Full-out fights, though, where blood is being drawn/fur is being pulled out, will require you to intervene. Try to watch carefully and separate them before a fight actually breaks out. If they do fight, give them a break for a week before starting meetings again.
One trick is to add a drop or two of vanilla extract to the dust bath they use (you can also buy scented dust bath) so that they all smell the same.