I have gone back and forth between rats and mice with some hatchlings and have never had any issues in the past. But with that being said, I have most definitely seen snakes that will only take ASF or mice and after seeing that, I will never switch back and forth. Sometimes I start the hatchlings with mice just to get them eager to eat, then switch to rats. But once they eat rats, I won't go back to mice anymore. I prefer rats and don't want to have to deal with mice PERIOD. I feel that most rat eating balls with gladly accept mice as a meal, but not all mouse eaters will takes rats. My suggestion is to just hold off, unless you're ok with the possibility of your snake becoming a mouser.