Well- I'm pretty sure MacGyver was never mentioned as having a middle name- so I would stick with Angus MacGyver. (After all, not everyone has a middle name- though it does make it confusing for the John Does of the world

)
And as far as MacGyver's rank in the Army- in the Mark Daniel novel "MacGyver on Ice", Pete looks through his file and it's mentioned that his Army rank was Major and that he worked with the bomb-disposal squad - and then it says his acting service rank was Colonel....
In fact, here's what's mentioned of his file verbatim from the book-
"Army rank: Major....bomb-disposal....acting service rank: Colonel....distinguishing marks: .22 bullet scar left shoulder, boar-tusk scar left side, clavicle to point of hip. Interests- military history, natural history, bebop, blue grass, horse-breeding, baseball, North American folklore, philosophy...."
But I don't know how much credence you want to put into the book- I'm not sure if anyone would really count it as MacGyver "canon"- though it is a MacGyver novel officially licensed by Paramount...
Anyway- hope that info helps.
It might also be a good idea to take a look at "The Lost Amadeus" as I know MacGyver pulls out his driver's license in that episode- and maybe you can get a closeup of it. (I know we've seen shots of his Phoenix ID before too- but I don't think it has his SS number on it.)