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?

Commissars before they can take the job have to serve a term as a Storm Trooper or Feild Police then go to OCS then a term as a Storm Trooper or Feild Police Officer then they go to Commissars School. ofcause they start their lives as Orphans in the Schola Progenium. atleast according to the Fluff, but some units "Police Their Own" and apoint (unoffichial) Commissars from their own seinor NCO's and Officers.

So are all the Player Commissars considered the unofficial kind? That strikes me as even more confusing than the Sergeant specialty not dealing with a specific rank.

So are all the Player Commissars considered the unofficial kind? That strikes me as even more confusing than the Sergeant specialty not dealing with a specific rank.

Don't know my self, they could be "Regemental Police" who are part of the Regiment and responcible for Dicipline and Good Order. or they could be Feild/Military  Police (That are part of the Commissarate) embeded in the Player unit, but I'd have to see the final copy of the rules to say one way or the other.

In the Table Top Commissars have always bean Heroic Profiled Characters (2nd ed M4, WS5, BS5, S4, T4, W2, I5, A2, Ld10, Immune to Psychology) (later ed's WS4, BS4, S3, T3, W2, I4, A2, Ld10) vs a normal Guardsmen (2nd ed M4, WS3, BS3, S3, T3, W1, I3, A1, Ld7) (later ed's WS3, BS3, S3,T3, W1, I3, A1, Ld7).

The homeworld stuff for the Commissars represents his experience with the regiment, rather than his actual place of origin. That’s how I view it and it fits established fluff; Gaunt, for example, took on many Tanith traits because of his long association with them. IMO.

The assumption with Commissars and Storm Troopers in OW is that they received normal training, but have since served long enough with the regiment to adopt their general customs and ways of acting.

Yeah, Schola Progenium characters are generated as usual due to having served for a long time with the regiment as has been pointed out. However, I personally will be giving anyone in my group who plays a character who grew up in the Schola Progenium the ability to talk, read and write High Gothic as well as Low Gothic. I know thay this is technically a free skill but in my mind it doesn't matter how ingratiated with the ways of the unit you're attached to, people with that background and in the positions they're in they wouldn't forget the languages that they've had meticulously drilled into them since childhood. It's more of a characterful addition than an in game breaker so I'm not worried but it does involve me playing a bit looser with the rules.