I love the fact that you feel like singing and playing the harmonica after rhinoplasty surgery! I also love the fact that you are so compliance-oriented that you are asking the question!
I would suggest that you obtain verification from your own surgeon, only because it is he or she that you will go to if you have a problem with over-aggressive harmonica playing and vigorous breathing (some possibly through an incompletely-healed nose), or belting out songs like the singers in Glee or Smash on TV!
Hum a little tune and gently coax some tunes from your harmonica and you'll be fine. But any more forceful tunesmithing should wait until you are able to be more vigorous with ALL activities, about 3-4 weeks. God bless and best wishes! Dr. Tholen