hmmm people who breed mice for show wait to breed the females until they are about 3-4 sometimes more months old but this is to enable them to be unmarred for showing...
however, i rely more on size if i seperate at all. I , like you would rather avoid the fights and most of my females do not get pregnant until they are ready...it is what would happen in the wild too...and the babies are not any smaller or unhealthy...in fact i notice that my babies are fatter and that my females have less complications...i have never lost a female to birth comlications. waiting too long to breed can cause more complications too, mice age alot fater than we do so a 6 month old female maybe in her prime for breeding but you will probably not get too many litters from her. I just seperate my show quality females for a bit longer than the rest but that is not a problem if you don't breed for show...Hope i helped