Depends on what you are calling a "rat pup." I have seen this term applied to 20 gram pink rats with a little bit of fuzz, and I have seen it applied to 30 gram furry rats. "Small mice" are generally around 10 grams and "large mice" can be up to 35 grams.

Petsmart sells the frozen rodents in little boxes right? Should say the weight of the product on the box, from which you can figure out if their large mice are similar size to their rat pups.

I would definitely switch if possible. I like to switch the babies to rats as soon as they are accepting FT food and are big enough to take a 20 g fuzzy rat.