I seem to recall that all the Commissars and Possibly storm troopers came from a different regiment as special attachments to regiment(Forgive me if that statement makes little sense). So a Commissar for example would not be from a regiment the rest of the PCs are from.
Is this at all reflected in the rules? does the character get to pick out his own regiment from which he hails? Or are all of the PCs from the same regiment regardless of Roleplaying Backstory?