There's not really a significant nutritional difference between rats and mice based on most sources. Still good to switch them early so you don't run into problems with having to feed multiple mice to an adult.

That being said, weighing your snake and then weighing the feeders and giving ones that are ~10-15% of the snakes weight is a pretty fool proof option for young ball pythons. It's excellent for people like me who are well, less than great at spatial things :/ If you have a bunch of frozen mice already, there's no issue with offering multiple mice that weigh ~10-15% of the snake's weight together. Meals of multiple prey items or larger prey items are (with some decent evidence) believed to be better than more frequent meals for most snake species. Digestion is relatively taxing. In at least one species, they've been shown to raise their body temperature by a degree or two during digestion solely through metabolic processes.