I have a baby RTB, I feed him 2 f/t fuzzies a week. How do i know when to change to say, frozen mice, baby mice, etc, whatever the next step may be?

Also, is it okay to feed the RTB in it's tank? or is it better to put him somewhere else and feed him, like a shoebox or something? I have Aspen as beeding, tonight was the first night i fed him in his tank, he didn't take in any of the Aspen, but is it okay if he sometime consumes some? ( obviously i'm going to try and make sure he doesn't ) but could that be a problem?

Thanks!