Dulahan said:
Honestly, this problem is writ large with all the support specialists.
Ogryns? They're not part of a normal platoon structure. They're their own units. (And I'll ignore my issues with how they are represented mechanically!)
Ratlings? OK, MAYBE. Some regiments. But not really, tend to be their own things. Still, one of only two that isn't too theme breaking most of the time.
Tech Priests? Maybe in a Tank Game… MAYBE. But they'd be helping far more than just the PC's tanks.
Stormtrooper? WTF? Seriously…. They make almost the least sense. With a caveat I'll go into below.
Psyker? Yeah, this is high level stuff too, not squad level.
Priest? This is the only other one that makes some sense, especially in a more pious regiment. Though I can't help but feel it would've been better represented by being a normal Guard Specialty called "Chaplain" instead.
Now, there are ways, but honestly an Enginseer, Psyker, and Commissar all dictate the campaign more than most things. It would mean your squad is the squad attached to said uber specialists as a sort of bodyguard unit. It is when this is the case there -might- be a justification for Storm Troopers. Say said PC was the only survivor of a unit, so attached to the 'key role' type as a bodyguard. (Especially Commissar and/or Psyker).
Alternately, that group - Commissar, Psyker, and Stormtrooper almost make for a super specialist unit on their own. And attaching a Priest to them as a command squad type thing almost makes sense. But that is still a very different game than a group of normal guardsmen.
You're right, to a point. Specialists really are not part of your standard Imperial Guard squad. However, Specialists are still part of the Imperial Guard and as such should be in the game.
To wit, Hammer Of the Emperor focuses on the "Guard" classes giving them new talents and advances - the Specialists get nothing from this book.
Specialists could be used as temporary/guest characters for when on of the Player Character's Guard character is medivac out.
Techpriests are pretty imortant for a setting that stresses the ignorance towards technology and where the average citizen lacks any mechanical aptitude.
If anything, I see problems with the Psyker and Abhumans based on the typical prejudices against those types of characters from the average Imperial Citizen. Sanctioned pysykers are a rare individuals and Abhumans are dying breeds based from the 40k cannon.
Only War gives you the tools to build the story. As the Storyteller and or the Group have final say what actually makes it into the game and more importantly how the group will play this game.