I'm with the others about not using any of the instruments in a Dynasty. It's possible to SuperMax, but it's very difficult. The Guitar Star journal challenge isn't worded quite right and what it really wants is for your Sim to complete 10 guitar opportunities. These will come to the skills opportunity slot and if your Sim has any other skills at all, it reduces the chance of getting guitar opportunities because the other skills will trigger opportunities, too. It's the same with drums, bass, and piano.