To answer your questions, yes and yes. Rats are much more nutritional for your bp as well as any snake...you will notice faster and better growth also.
And it is easier to switch when younger because once they get established on mice they may not switch ever though very rarely. BP's can be very picky eaters. If even at this age or if you choose to wait and you have a problem switching try rubbing the rat on a mouse and the scent may get the BP to take it. I feed all of my BP's and RTB's rats from neonates( rat pinkys), sub-adults (rat pups or weanlings), and adults small to medium rats.