I believe rat pups have a bigher fat content than mice. However, if you are only feeding once or twice till you can get some mice, then I would not think this would be a problem.
Mhmm, snakes have excellent stores in them they can go for months without food, as said in previous posts, they don't know when a rat is gonna come along.
it might do him a bit of good. keep him active when he's "hunting"