Brigadoon is such a great show! I was in it a few years ago.
Tommy, Jeff and Andrew are all very different parts. (And don't forget Harry!) You should definitely review the StageAgent page on Brigadoon & associated character descriptions before you audition (http://stageagent.com/Shows/View/700
If you auditions for Tommy or Jeff, I recommend that you choose a comedic monologue. For Harry, choose a dramatic monologue. For Andrew, I'd go with a Classical or Comedic monologue. Just look through the StageAgent Monologue database (http://stageagent.com/Shows/Monologues/
) and you should find a monologue that works. For the age range, choose a monologue that is within the age range of the character.
Break a leg!