Hopper mouse or fuzzy rat

I agree with Brandon. Interestingly enough one of the problems with feeding snakes can be their liking/disliking a certain food. I've heard they're more used to gerbils (native although I'm sure this is more instinctual for the CBs) also when I had some troubles with my male earlier this year I tried switching fur colours a few times (he tends to like them darker). So I would wait until your snakes are big enough for small fuzz/hopper rats rather than mixing them in with the mice, the transition will probably be easier than you'd think :D

Well, ok, it can wait for a while then.

The whole rats have more nutrition than mice is mainly myth... there was a published report from a zoo on the components of various food items. Mice are actually richer in calcium and much lower in fat, while having marginally less protein. It was posted here a few months back so perhaps you can try searching for it. You will have to make the switch over to rats eventually, but I would wait until your guy is nice and fat and can afford to miss a few meals before making him switch.

Definitely start feeding larger mice though... it's been my experience that ball pythons can take prey items as large as 1/3rd of their weight, with 1/4 or so being ideal. Also, the younger the ball python, the larger the prey item they can handle in proportion to their weight. If you think about it, prey items in the wild don't exactly come available in the convenient sizes, so younger bp's need to be opportunistic. Not to mention, younger ball pythons have proportionally larger heads.

So, even a small hatchling should take juvenile mice. You should either feed juveniles or larger, depending on the size of your ball python.

Here's the study I think hhw is referring to that I posted a few weeks ago.