Community selection (fighters, heavies, helos) is made during UPT, not at the commissioning source. Although it's likely that there are less flying spots right now, which means ROTC/OTC/USMMA people will have less of a chance of getting a slot than the AFA cadet.
On another note, you should be asking yourself if USMMA really is the right choice if you want to fly fighters for the Air Force. In order to commission into any of the Services you need a certain GPA and availability. For instance, I seriously doubt that the Air Force is taking anyone from USMMA because they are already cutting AFROTC scholarships, canceled OTC for this year, and are reducing the number of AFA cadets for '15.
It's really going to suck if you want to be in the Air Force but are stuck studying the maritime industry.
Sam