Why not use MIDI format for the musics?
It's more or less recognizable when playing the music, but it can get you out of the copyright stuff...
There are ways in which to configure a RockBand guitar for DBPro, but you're going to have to test and query and test and query the buttons to see which ones are working on the neck of the guitar controller; the start and back buttons, and the button pairs and triplets (i.e. green+red, green+blue, green+yellow, green+orange, green+yellow+orange, etc.), the strum bar, and so on.
There was a program out there that you could tweak, using a Guitar Hero controller, that you could actually play music with... I had it set up almost like guitar-bells and nearly had the baseline to Led Zeppelin's "Stairway to Heaven" down!
Also, there's a set of tools that you can use to create your own songs for RockBand, as part of the RockBand Network member... It's free and its available for download, as RBN is still looking for new songs and bands. (The name is escaping me... #>_<
There are programs that can actually convert what you sing into MIDI. (Music Masterworks is one such program.) This way you can record into a good USB microphone (Logitech), and sing the parts of the song that you need: percussion, lead guitar, bass guitar, keyboard, voice harmonies...
These are just some of the things I have used for various odd projects. I hope this helps.