It's not that difficult. It just goes "Fire, fire, reload". Now, I think it's fairly difficult to tell them to reload after each shot while using combat shotguns, without making them to reload after each shot while using things like chainguns or plasma rifle. Why'd they even reload after each shot if they could just pump you full of shells instead? It goes against the common sense of demonic zombified creatures, I'd imagine.
We'd have one right out of the first level, yes, but the power difference is still against you. Remember that there're loads of former sergeants out there, who have the same small spread you do. It's one against million, like it's supposed to be in Doom. And what if you don't plan to use shotgun at all? If you wish to use chaingun or something else, the game's suddenly become way too difficult for you.
In the original game, the player could take much more damage compared to the enemies, and the sergeants didn't use shotgun in its full power, unless maybe if you played on more higher difficulty levels. It didn't do much damage to you, but you could still rip things with the same weapon. If you wanted sergeants to wield combat shotguns like in the original, you'd also have to make it do less damage in their hands, which is once again much too difficult for its worth.
You make a good point with Hell's Arena, but remember that the former sergeants don't just suddenly become extinct after that. They're still out there, ripping your armor and hit points apart with their much too powerful combat shotguns.
Here's a compromise, though: Let's make it possible for them to pick up a combat shotgun and use it instead.