I was told (not sure how true as no one gave sources to studies which would have been good to see) that boas tend to be prone to having issues digesting the milk in the bellies of baby rats and it can cause regurgitation.
I'm guessing proportionally the hopper mice have less milk than similar sized rats as they're usually nibbling real food by then and have smaller stomachs so it causes fewer issues.
I know BCC/BC are more sensitive in general than BCI/BI, so it might be a bigger issue there. I swapped mine (BI) to rats at the large pup size for convenience and have had no issues with my 2 boas. I probably wouldn't want to do fuzzies, the pups are usually eating some real food already so it isn't as much milk to process.
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