Since you have a rack the size doesn't matter except for how soon you want to start producing. They can breed as young as 5 weeks but you will have small litters, if they even survive, and a lot of energy that should have gone towards growing the female out will instead go to the babies. I usually wait until my females are around 7 months but go more by the size of the female than her actual age. Try to get stock from healthy bloodlines that produce large litters. You may even want to get some females that are around 1 year old and have a litter under their belt already to increase your chances of a smooth pregnancy and rearing right from the get go.