Adult mice are fine to feed, the only issue is when it becomes multiple mice vs 1 rat and the cost goes up. Nutritionally either way is fine. (I crunched numbers a while back on the nutrition off the study from rodent pro's website, they're not significantly or concerning different if you feed enough mice to weigh the same as the rat they should be taking.)
Switching to rats is cheaper, but many young BPs are intimidated by rats, either size/shape or smell there's no telling what is really spooking them. Best bet would be to thaw one of each together in a baggie to scent the rat lik a mouse and offer the rat. (Or "borrow" stinky mouse bedding from a pet store.. either one works.)
I wouldn't feed both that are thawed since it would be too much at once so you may waste a few in the process even if he takes it and at his size, I would probably not force it personally by skipping meals until he accepts a rat (which I'd do for ones approaching 250-300g where they are outgrowing adult mice). If he refuses a rat scented like a mouse, you can offer the mouse with some rat scent on it. Even that slight scenting may get him familiar enough in a few weeks to take the mouse-scented rat after a few tries assuming he will take the mouse that has the rat scent.
No harm in feeding the adult mice though until he's a bit bigger and has the weight so you can skip 2 or 3 meals if/when he refuses the rat. (At that point I'd give a mouse to keep him from losing weight then repeat a few more weeks until he takes the rat.)
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