I think Brandon is assuming you are using the same cages for grow-outs as you are for your breeders. In this case, it is easy to see how you'd need one per each breeding group. If, however, you are running relatively small breeding groups, and putting all your weanlings into one much larger grow out tub, there should be no problem with combining the offspring from different groups.

What I have been doing, with virtually no fighting, is when it is cage cleaning time, I put the ready to be weaned mice from their mother's breeding group, sort them by sex, and put all of one sex in a holding tub. Then I take the mice from the mice from the appropriate grow-out tub and add them in to the holding tub. I clean the grow-out tub, rearrange the furniture (for example, if the wheel was by the water, I'll put it by the food instead), and add all the mice back in.

I've had mice that were at least 8 weeks old mixed with newly weaned mice with no difficulty at all. One thing I recommend avoiding is having a cage with only 2 males. Larger groups of males (with no females) have done just fine for me, but the one time I had only 2 males together, they fought until one was so badly injured I had to euthanize it.