Well, I finally bit the bullet and filled out a chart with what I wanted to be in my Trinary to play as the Scorpions. I started with a Guard Trinary, as it looked the closest to my intention of 'Garrison second-line troops.' I swapped out one of the elemental stars for a vehicle star, since none of the other formations listed had vehicles. Also, I interpreted the 'elemental' star I retained to mean, 'Roll on the infantry column 5 times.'
I actually did roll the chart, though I made some substitutions based on what would make sense for a second line/solahma unit, as well as based on what things I thought I could represent with the toys I have.
With the mechs in the Attack Star, for instance, when I rolled an advanced omnimech, I would choose a second line mech in the same weight class. This is how I got the Charger and Orion. These were mechs I wanted and thought I could represent. I kept the Hellion, because, well, it's not that great a mech, and could easily represent an unpopular captured machine being kicked to garrison service.
In the Support star, since I had ten vehicles to choose (since two vehicles for a point, unlike with mechs), I had to do substitutions just to make up unit count with things I could represent. I actually rolled the Mithras, Ares, Von Luckner, Oro, and the APCs. I think I can represent those, and they're nothing unreasonable for second line. I swapped in Scorpions and Pos to replace better omnivehicles that I felt were inappropriate or that I couldn't represent with the toys on hand.
For the Garrison star, I just let the chips fall where they may. I'll have to find ways to represent these oddball units, and I frankly don't have much idea where to start. I actually rolled a protomech, which while good for power, is hard to wrap my mind around for modeling. The infantry platoons are easy. I'll use army men. I also rolled a Horse mounted infantry platoon, which I have no leads on, and an elemental point, which...will bear some thinking about. I'm always open to ideas, so let me know if you come up with something for these units which I have not thought of!