Brad Pitt would be a little too old to play Scott in his 20's, since Brad's 45!, same with Bacon and Madsen, what's up with the old guys folks?
I'd say Robert Downey Jr aswell for Scott, Russell Crowe as Eddie Vedder, Vince Vaughn as Robert, Owen Wilson as Eric Kretz, Ron Moss as Dean Deleo (hey, similar jaws...)
Geena Davis as Dave Mustaine (since STP's first big tour was supporting Megadeth, that would surely be displayed)