Sorry, I missed the question you asked if you could play with just a guitar. Yes you can, just make sure you start the game with the wiimote plugged into the guitar otherwise it will assume you are trying to use a mic.
There's no I in TEAM but there's a ME, and there's no I in TEAMWORK but there's an EWOK. Zorak
i dont think you could use the wiimote anymore to play guitar you need the controller because i read the whole book nothing about the wii mote being used for guitar/bass. so you need mic plugged in plus even if you put the mic when you go to quickplay it say vocals only the other instruments dont show.