We started to bulk breed the rats here since we have switched most of our sub-adults to rat pinkies. I put 1.3 or 1.4 rats in a concrete tub.(rack system) It seems a little cramped but the "objective" is to not allow the female a chance to fight off the male. (Often you will see male and female wrestle before mating) We have seperate bins(x-large cat litter bins) to place mothers in when they start showing that they are pregnant. We use to keep the males and females together in a 1.2 colony but we had a male eat a couple of litters just because it brings the females back into cycle. Once the mother has the litter, We cull what we need when they reach the right size. I then give her a day or two on her own, to eat and recover from the stress of having to feed so many. Then she will go back in with her group, which at the time could be just the male. Rats are different than mice, as they are a social animals, so re-introductions are often smooth and without conflict. We feed our rats a mix of pig food, seed and fruits and veggies. We also offer a cattle cube or two for them to chew and eat on. This cube is real high in protein and fat,for those nursing mothers, but it is not a substitute for the mix.
Most rats are "around"(give or take a week or two) 3 months of age before they start breeding.