First off i leave my mice and rat breeders together at all times.The males bring food to the females.Second if your corn snakes are adults then they can take a rat fuzzy.I have three breeder females and they eat a nice size rat fuzzy with no problems.Plus rats have more meat on them then mice plus they do not stink as bad .I would not even try to feed gerbils to anything.The way i kill my rodents is i take them in my hand(welders gloves on) and squeeze a pair of pliers over thier necks.Clean and no suffering as they die instantly.