I'd use all 4 fingers and
practice, practice, practice
You will hit many brick walls on your way to Guitar Godness and think you will never overcome them but the more you play the better you get.
When playing hard if I get stuck I go up to Expert and play the easier songs and then go back down to hard later. If you continue to fail take a break and go back to it later, it always helps me. Above all don't think its useless trying anymore and give up, once you get the hang of it, its a very rewarding game and probably the most fun I've had on the xbox
There are also plenty of GH / RB games out there so if you do get stuck (and have the funds) grab yourself GH2, 3, Aerosmith, RB or RB2 and have a lpay through those. They will bring back the fun factor with all fresh songs and give you that much extra practice.
Good Luck