Sorry if I used big words! :)
As far as HT's went when I was active duty, advancement was very quick for the most part. The HT's that enlisted with guaranteed schools because of higher ASVAB scores would probably have a slightly better chance on doing well on advancement exams.
When my enlistment came to an end, I requested NDT School. I was an E-6 and they wouldn't allow it because it was considered a "conflict of interest" because I was a Nuc welder. Never did figure that one out. The other thing was that if I were to make E-7, there was no assurance I would keep on welding so I was done.
How things are done now, I have no clue since my time was many years ago.
Regardless, getting all of the "details" worked out before signing up is important but recruiters are really only a notch or two from car dealers.
If the job to be considered is only a small percentage of the group, then that could be a positive or negative thing.
Positive, you are "special".
Negative, advancement is slow and trained and experienced peers to work with are few. If there were only 100 1316's in the Marines then I imagine welding was not a big demand skill requiring welding daily.
If someone wants to go into the military there are many choices. I consider the Marines to be best at what they do, the Navy best at what they do and so on.
Regardless, nothing beats working for less than minimum wage at a job you can't just "quit" for a boss that may or may not be any count.