Had to feed my girls adult mice while waiting for my first rat pup order to ship, and they had no problem with either. From what I understand, most breeder start hatchlings with mouse hoppers because the movement entices them to strike, then they switch to rat pups once the babies have taken three meals and developed a good feeding response. I think it comes down to the individual snake's preference whether or not they're a mouser, and with most you should be able to switch with persistence and a few tricks.