I view AT-AT's as a team myself like tank crews similar to Paul's analogy.
EU says there is 2 crewmen, a commander (Officer?), and then the troop complements in the back either have their field officers or there's AT-AT crewmen on each deck to "motivate" the troops into getting out.
They could the troops own officers or part of the walker's crew though. I think it's more the prior than the latter.
As for the Commander, I envision an Officer-ish looking guy. Maybe the rank's lower, but the outfit's the same, so the Veers figure is the basic look but the rank just changes.
Veers' armor/helmet is supposedly the Imperial Army Regular standard... Some EU shows them wearing a Y-Harness get-up too over the AT-ST Driver's uniform, and then the Veers armor/helmet is their just additional armor over the gear. I envision officers being just like Veers though, but with differences in the badge.
So to me, AT-AT's are manned by a team or crew, one officer (generally low rank) with 2 crewmen, as I see it. They'd then be grouped into armor columns/squads I guess.