Why not just give penals the 7man upgrade at T4 to solve their lack of late game. It would even have synergy with their vet 1 ability " To the last man ". So in the end, soviets are left with a choice between 7 man cons which have mediocre firepower but great utility or 7 man penals which have good firepower but bad utility for their late game infantry.
Have you thought about making T3 optional and not required for T4. Instead, to unlock T4 you need 2 of the previous tiers. For example, you could go T1+T2 into T4( I think this wouldn't be too good since you are sacrificing the mid-game presence with t70 to rush out the t34 ). This could also be an indirect buff to penals since you don't need to backtech to get the ZIS gun or Maxim.
I would say its a downgrade. Partisans are best used ambushing from camo in close range with their submachine gun( not sure if they use ppsh or mp40 ) for a quick burst. SVT would make this tactic awkward.