If you secure a position with an early match, you have to withdraw from the NRMP match. (exceptions being optho which requires an internship to be obtained via NRMP)
Each of the early matches have their own scramble I believe.
People might not be interested in any of the early match specialties. They are rather competitive and to be competitive for the match students usually do research and audition (away) rotations. If you are applying just to see (without the preparation work), the chances are going to be pretty low that you'll match. Might as well throw your application money down a hole.