As long as they get it down...it's all good.
I've heard that rats have more meat to them so it's a better meal for a BP than a mouse. Mine is still on mice though.
I don't know for sure but I read that there are no "mice" in wester Africa where they come from. That's why it's sometimes hard to get BPs to eat mice. They do have gerbils and such so they eat them up but it's easier for us to feed mice/rats...so we do.
If there is anything the nonconformist hates worse than a conformist, it's another nonconformist who doesn't conform to the prevailing standard of nonconformity.