Under the current rules, if your group wants to play without comrades, they could try playing a stormtrooper fireteam. Or stormtroopers lead by a Commisair. Or why not 4 Commisairs? The rules have nothing against that.
To be honest though, I feel like Comrades are a huge cop out. The arguements I hear is that, they do the heavy lifting so that the players can do 'heroic' things, or that the comrades are cannon fodder, or that it fleshes out the squad.
First of all, if we wanted to play heroic characters, we could just play deathwatch. If you play IG, you're doing pretty much the least glorified thing possible. If you're going to seriously RP that, you're supposed to do that heavy lifting, GRUNT work yourself.
Secondly, the squad can be fleshed out by just adding NPCs. Are the players playing an infantry squad? Why don't the DM just add 5 or 6 NPCs to fill out the squad. Then, if none of the players play a heavy gunner, and the squad might come against a tank, the DM can just add an NPC heavy gunner with a rocket launcher. Same thing with weapon specialist or sergeant. Is the squad playing a weapon specialist or heavy weapons squad? Those are capped at 6 guardsmen, so 4 players with comrades already break that canon.
Lastly, the fact that comrades reduce the danger of being shot is the biggest cop out, ever. Being in guard has it's dangers, and by skirting around this, the players are losing out on the exact RP opportunity that being IG is supposed to afford them, as opposed to say dark heresy or rogue trader. Being a guardmans is dangerous, but you have protection in numbers. Not just your squad, but your platoon, and then your company. Comrades doesn't take account for that. Just adding more NPCs will.