Where can I find information on MT and HE mechanics, like the job requirements at the different echelon levels, job responsibilities, etc.?

I here to tell you mechanics in military in the Grade E5 or Above that either work second Echelon at company Level or 3rd or Depot mechanics. I was a 63B30 I went to many military Schools and College and I work and replaced Engines Transmissions and Transfer cases Work on all Equipment Generators Forklifts Track Vechicles and More.
SSGT MARK FRANZEN
USA VET

From the Army, you are likely looking for Mechanics that worked in Support Maintenance (3rd Shop) as front line mechanics are pretty much limited to mostly preventative maintenance and consumable part replacements (Tires, filters, Oil changes and the like). Support maintenance gets into major repairs in the engine drive train et al so those are likely the skills you are looking for.

You might also want armor or heavy equipment mechanics as they would likely have more hydraulics experience.

For CM's in the Seabees E-4 & E-5 are usually crew leaders. E-6 shop supervisor or even maintenance chief depending on the size of the detachment. I was a maintenance chief responsible for over 200 pieces of equipment spread out over Kuwait and Iraq as an E-6. When I picked up Chief, I was an Ops Chief and a Detachment AOIC. In the Navy/USMC family, we tend to give our younger folks a lot more responsibility than our Army/Air Force counterparts.