There's tons of differing opinions on how to keep reptiles.

If you have confidence in a certain way, then you shoudl definitely go with it. I personally think this site has decades of experiance between the members and it's freely given.

She believes whatever she wants to believe, and as long as she sells you the prey item you want(ie rats) then nod and repeat "I'd prefer to feed rat pups, thank you." and let her think whatever she likes.

I'm unsure how she thinks mice are bigger anyway. You'll be moving the size of the rats up as your snake grows(keeping them the same size as the largest portion of your BP's body), while mice will soon be too small for a decent BP without feeding them multiple mice each feeding.

Shrug. To each their own, right?