I actually prefer to leave the Sergeant as is for the following reasons:
-At the start of a campaign the players probably aren't going to be part of a command squad. Keep in mind they only have a logistics rating of 10 to begin. This is in line with them being enlisted men.
-In the table top game a squad of troops is lead by Sergeants/Veteran Sergeants. While I suspect your argument is based on how it is in the US military, this is not the theme of this RPG. Any confusion about what a Sergeant character does can be cleared up simply by reading the specialties description.
-The kind of officer you are describing would have more in common with loner characters like the Commissars and Stormtroopers and would require a reworking of the character concept.
Point 1: Unless they're tech priests, priests, Sentinels, Commisars, snipers, armored just about anything, or heavy support just about anything. Since all those report to the command squad, not the gopos they're attached to.
Point 2: So you're saying that the theme of this game is that Sergeants spring up out of the ground and promotions never ever happen in the rank and file?
Does not just fly in the face of logic, it flies in the face of fluff (After all, the 112th Rough Riders were being lead by a former sergeant turned colonel following the fighting against the tyranids on Coriana.
Point 3: not seeing the reason it would require that level of rework. Unless there's some reason from GW that it would, I know they've been a pain in the ass for approving things.
After all, it's Cain/Gaunt/Yarrick that they want to play, not some random asshat that stands around shooting other members of the party in the head for cowardice.
Oh, and incidentally, all three of those Comissars are melee monsters in their own right. And by that order of proficiency.
Actually it would be Gaunt/Cain/Yarrick.
As far as all three being exceptions, the only one who actually is would be Gaunt, due to his position. There's this weird thing I've noticed in 40k that anything that does not agree with the speakers point of view is 'an exception' even if,in reality, they're basing their entire position off one line in one codex three editions ago and the bulk of the examples make that the actual 'exception'. (We ran into this problem with Space Marines and lances when Betaing BFG FAQ 2010. There was an absolutely rabid response that SM could not ever have lances ever because there was a line in BFG armada that stated that the IN and IQ were concerned over the Nova class frigates superior performance as a capital ship killer, and it was armed with a lance. Never mind it was faster and more agile than IN frigs as well as packing that F/L/R lance, it was obviously just the lance they were talking about. Despite the fact that a dozen sources could be brought from BL and Codecies that said SM had lances on strike cruisers)