While I can't confirm it, I think Twallan has a mod somewhere that converts the Guitar metric in the music career into one called "Musical Skill". That metric uses the same numbers as Guitar does, but takes into account the highest level of any of the bass, piano or drum skills as well as guitar.
This can make it so that your Sim is a pianist and can still get far in the music career.
EDIT: "Musical Skill" includes the Laser Rhythm-a-con from Into the Future too; perfect fit for a Hit Movie Composer.