I think trying a protein supplement or any dietary supplement is fine if you monitor your use. If you are not getting the results you want, stop using it.
It doesn't sound like your workout regimen is geared toward bulking up, so I would feel fine trying a protein supplement without the fear of building bulky muscles. Just keep monitoring your shoulders (measure them if you like) and make adjustments to your diet/workout regimen.
I have just recently began to eat a PowerBar w/protein before my workouts. I think it may make a slight improvement in my energy and help me to maintain some of the muscle I have during my dieting.