Be careful. If you feed gerbils you may find you can't switch em back. If you are worried about the size of rats, just feed them off before they reach that size. If you have to many whack and freeze them. I think most people have an easier time breeding the rats, but you may want to try the search funtion because I believe we have a bunch of threads on breeding rodents of all sorts. What are you feeding them? Maybe it's diet related?

(I also moved this thread to the feeder section, since it's more about the rodents than the bps )