While indeed there were former commandos in the original Dooms, we knew them as the lovable blokes with bullets for our chainguns. Now they murder us with plasma rifles. And thats not too far?
(vague agreeing with Melon's statement :P)
For my money, new monsters are fine as long as they fit in with the existing cast and don't actually replace or cause the removal of an existing monster and indeed, what was the commando (or "heavy weapon dude" if you prefer) in the original game simply became the former captain, while the commando became an HWD with a plasma rifle, something I have no problem with... And why shouldn't there be a few elite formers who are able to play with the big boys? Maybe there should be a former rocket trooper as well...
Anyway, I very much like both the "Shotstol" (though it
desperately needs a better name (though the best I can come up with for the moment sadly is sawn-off shotgun)) and the former security officer ideas, though I think plasma and explosive type weapons should be able to "lance through" the shield, causing the SO to take half-damage since, well, let's face it, a former security officer being completely immune to a BFG shot or rocket explosion (the concusive force alone of a high explosive round is considerable) because of a little portable piece of metal seems a little hard to swallow...
Going back to the rocket trooper concept though, that actually gives me a different idea...
Former engineer:
This combat engineer's inventive mind sadly wasn't enough to save him from the demonic hordes... His brain (or what's left of it) now works for them...
Essentially, this former would manifest himself as a green or yellow "h" who would start off with no weapons at all other than punching and perhaps the same Glock the former security officer would use, but would be able to pick up and use any weapon he can lay his hands on (including uniques), meaning that the player could, concievably, find himself staring down the barrel of a nuclear BFG wielded by a former engineer... This would, however, necessitate the player being able to tell what weapon the engineer is carrying and would also necessitate some method by which the engineer could chose which of his carried weapons he'd like to use, all of which is perhaps too complex to be implimented effectively...