Being an SSO has absolutely nothing with joining any of the unions and will neither harm nor help you. If you do decide to be in that program keep in the back of your mind that many of the classes taken at the respective schools will earn you points. At least that is the case at the MEBA school so I assume it is true for the others. If I recall correctly you need something like 50 points to earn a “retirement” year. (I could be wrong on the number) Points are earned on active duty training, courses taken, weekend drills etc.
I leave it to others who are a SSO, to add comment.