I think having some extra tutorials for the games that haven't been in Megamix before (i.e. Rockers, mostly) would be very helpful. The best I can glean from it is it involves holding whatevers set as the A button for the length of the riff, using what's set as the L button for bends, but it's very inconsistent. I might also recommend using some sort of calibration to account for HD resolution and wireless controllers. That kind of thing is EXTREMELY necessary in any sort of rhythm PC game. I also suffered from a lot of slowdown in the somen game, for some reason.
I really appreciate the effort put into making this, and as soon as I saw someone play it I knew I had to download it myself. I hope my critique is constructive, and I hope that this program improves!