1. The really important thing is what faction you are facing in this question. Sniper is very effective against MG and Grenadiers. Guards can really hammer OKW for various reasons. The slight CQC bias of Strums and STG Volks and the use of light tanks. Some adaptation but a good baseline. Mortar or sniper is whatever you need to counter. Sniper is going to be a micro tax. If you think another unit will be more effective then a sniper then consider skipping/postponing it.
2. If you are really big on counters(which it sounds like with your build style) I would recommend trying a doctrine with radio intercept. If you hear there is a 222 then guards, if you hear there is a sniper you can try to go counter sniper very easily with the element of surprise. Personally I find Penal builds slow early game so I don't love Armored Assault for them as it is extremely late game heavy(ignoring the early game benefits of radio intercept of course).
3. PPShs for Penals are pretty decent. They should be compared to shocks. First off Penals with PPSh come at 1cp vs 2 for shocks. The cost is 300mp/45muni vs 360mp. Reinforcement cost is higher for the shocks 24 vs 34 iirc and they are more frequently used abilities resulting in more muni use. The other aspect is you can merge conscripts in with either squad to make them cost 17-20mp to reinforce with a notable hit to durability. 10% easier to hit if you merge cons with penals, and 10% easier to hit and without any of the shock armor if you merge with shocks. Both you can dodge mp bleed if you have too by merging with cons. Moral of the story? Try penals with ppsh if you want early harassment or just simply one of the best CQC infantry. Merge with conscripts to keep yourself in the fight if manpower is not going your way. Against OKW a scout car with ppsh penals can be very deadly. If you are interested in airborne commander I recommend this video from 1 month ago.
https://youtu.be/bWSjCZqb1Uo The other thing to consider is the veterancy. Penals have better combat vets while Shocks have much better grenades, durability especially with vet. If you find yourself using grenades a lot or fighting on open maps then shocks are going to be absolutely to be better. There is no right answer. My case is if you want PPSh Penals you should pick a shock commander since shock commanders are better then Penal PPSh commander.
4. T3 and T70 is the bread and butter of Soviets. Especially with Penal PRTS. The Penals with PTRS are fairly toothless against infantry. And sadly worse then guards against tanks unless you use sight blockers to throw at satchels at a tank. I have gotten 3 at satchels off this way on players up to around level 13-14. T70 can mop infantry while guards, ptrs penals, and ZiS guns end tanks. T70 will either flat out win you the game or allow you to stay in very long. I wouldn't skip T3.
As for finding videos; Propagandacast always gets some interesting Soviet games. Or just spectate. When I have down time I just watch the spectate list and drop what I am doing if I see a top player playing a faction I like. I find it is good tactical map practice and watching resources.
Just experiment
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