I'm spitballing here, but what if the SU-76 had a Target Turret ability that renders a critical hit to the Turret so that it gets locked? Tigers would still be able to fire and move normally, but if the turret was locked at 6-o-clock, that means it would have to present it's rear armour to an enemy in order to fire. One can argue this is a ripoff of OKW Puma's Aimed Shot, and it is, but then again USF's M10's Flank Speed is a direct ripoff of OKW's Combat Blitz, which itself is a ripoff of Ostheer's Blitzkrieg.
For the ZiS-6, perhaps a suitable support role is making it a stun ability, but used like an HE barrage: basically a single shot fired at a target area, and everything in the vicinity is stunned, infantry to vehicles, friend or foe. Kind of like a universal stun grenade. The effects are temporary, maybe as short as 2 seconds, and damage would be low-to-none. For vehicles it could be a one-shot version of Button vehicle: the target is momentarily slowed and blind. Guards' Button would last much longer (though frankly given how terrible they are they won't be much use against Tigers and Panthers).
Camouflage for M-42 would be nice, the Racketen has automatic camo at like vet2, even when in the open, which is retardedly OP. A doctrinal unit that performs far less getting camouflage at vet1 isn't even near getting towards balanced. For a low-tier plinker camouflage is very necessary, if not most suitable to the game's smaller gun.
A Canister shot similar to Greyhound would also be nice as an alternative, though it also seems amazingly accurate against infantry. Or perhaps one of the above mentioned ideas. the M-42 seems to be a perfect support unit for the rest of the Soviet units, and tackling heavy armour to allow your other AT guns and such to destroy it makes the M-42 a worthy investment, even if it by defaults grants low damage. M-42 is found in only two commanders, and both don't offer much in terms of AT firepower besides this unit, so stunning/ camouflage is a good compromise to just buffing them.