Hi Jennifer!  I love your "out of the box" casting brain! I have done it several times with girls playing the gangster roles, complete with the classic suits etc., but never cast a girl as one of that trio.  Most likely you have a strong girl vocal that will add some security to the Fugue which is great.  My only concern is being such an iconic role, that it balances the other guys and doesn't over power the main female leads. That can be solved by good artistic direction.  Anxious to hear from someone who has done it. Forge ahead!!