Got another question along the same lines..only when is it ok to breed a female? The same 6-8 weeks?
When one of my older females gets past breeding age and I retire her, I generally pick the healthiest female fuzzy currently being nursed and just pick around her when feeding days come. That way she grows up with that particular colony and I don't have to let her duke it out on the pecking order.
So far, after a couple of years of doing this I haven't had a problem. The females breed when they can and voila, babies all nice and healthy. The females that have done this don't seem to be showing any health issues related to it at all. Then again, I don't have an x-ray machine nor a veterinary license.
In the beginning I tried to separate the females in a separate tank and let them "age" up a little bit. I used to raise rabbits, and it was hard on young young females to just start out and pop out babies. But I found when they were away from their intended colony for more than a week, re-introductions were rather violent. And it put the new female in such a state, that she wouldn't breed for a month or more.
Maybe I'll try the re-separation thing to see if I can get my females healthier and more prepared and just not worry about the subsequent fights that'll break out.