While I realize it's far to late to change, I think no pauldron should be private and black pauldron should be assistant squad leader or Corporal. When I was a cadet, pauldrons were only issued to NCOs. You don't need to put a pauldron on the lowest rank. They are easy to recognize as the guys without them.
Alternatively, they could be used to mark Privates assigned to a specific task. I.E. the Privates in Platoon One wear no pauldrons and those in Platoon Two wear black. Third Platoon wears grey, etc. With this route, white would make sense as assistant squad leader and orange as Sargeant. The problem with orange as Captain is that every third TD in the film seemed to be a Captain. Even as Sargeant, that's a bit of a stretch.
Darn. I overthought this, didn't I?