"Does inclusion in the Radio category supersede the Music category? If not, I don't understand why it was rejected from that category."
Radio is more specific than Music, but no category supersedes another. And Radio is the right category, because they call themselves Slacker Radio. No need to tag it with Music too.
https://domain.opendns.com/slacker.com
"Also, is there a way to request that a category be reevaluated for tagging to a particular site?"
Sure, visiting the link above and clicking "Flag for Review". Domain tagging is community effort. Or open a support ticket.
