I've always had 2 or 3 boys together since my daughter was around 4 and is now like 27. Never had issues with them getting along. This is the first time I've had girls. This is also the first time I've had a lot. I have 8 boys, all brothers. 2 groups of 2, i group of 3 and a single. The single is very aggressive to his brothers. One of the groups of 2 are both very timid boys who cling to each other and their humans, but the rougher boys tend to frighten them. The other group of 2 I called the grouches. They don't like being woken up, they don't like things changed in their cage..they are just grumps that do well and respect each other but their brothers irritate them moving blankets and other bothersome things..like sleeping in their spaces ect. The group of 3 are my rough boys. Sweet, but play hard, tend to have a few displays of dominance from time to time..but get along great with each other most of the time.
Other than my single boy, the rest used to all be in one big group, but between 9 and 12 months the more dominant attitudes arose. I just spent a lot of time watching behavior and then split them accordingly, which seems to of done the trick.