I seem to recall that Double Blind has all the non-Command MechWarriors that show up as Ensigns, but I don't have the pages to back that up right now (nor can I swear that those Ensigns aren't lance commanders or something, but I don't recall them being that way). And I'm pretty sure that all the aerospace fighter pilots mentioned are ensigns as well.
Of course, with the Magistracy, that could just indicate that they were able to purchase the Ensign rank, rather than it being standard practice.
And the odd ranking is something I wish HB:MPS had cleared up a bit.
Oh! And I think I might have figured out where the 16 regiments for the MAF might have come from. The General of each Combat Formation has a company of `Mechs for a personal unit, so that's another 5 Companies of troops (Royal Guard, Fusilier, Light Horse, Highlander, Cuirassier), which might just be enough to bump it up from 15 to 16.