I believe 18X is for non-prior service people -- high school or college kids and people off the street. For the Guard, a REP-63 general-MOS enlistee is pretty much the same.
I may be wrong, but the National Guard (especially a National Guard Special Forces UNIT recruiter) gets credit for filling a short-strength MOS (especially Medics because it's our longest course and has very high academic standards). If a unit can't be manned it'll be moved or re-flagged to a different branch.
I was in an airborne unit that was inactivated and given the choice to change to a different unit (in my MOS) or stay in-town to become an armor cavalryman. I stayed infantry and volunteered and was accepted into a Ranger company.
SF training is a long road for either active OR Guard. A Guardsman has to go through jump school, Selection, then SF qualification training -- it can take a while. While you have some flexibility, you will serve at the needs of the service -- you go when they have a class and opening, and you'll be away from your civilian job and family.
When I taught the Q-Course, Guardsmen and Reservists had to maintain the same standards as active duty.
I know a number of guys who separated from the Marines, passed the Q-Course, and went on to do good things in (active duty) SF.