Just when I thought everyone had asked every possible question about HOI4, someone comes along and asks about a US versus USSR game. I don't have all the answers, but I will try to give some:
With a friend we planned to try MP and we've chosen USA and USSR. I thought a good home rule would be to deny USA to join any side before USSR is at war so if USSR choose to get rid of the purges by attacking let's say iran, USA would be able to join/declare war when possible, if not, USA enters the war when Germany attacks USSR or USSR declare war to finland f.e.
If the USA is allowed to join the war the moment Germany attacks the Soviet Union
or when the Soviets attack Finland (or some other target), then we have an interesting situation. Normally, the Soviet Union wants to increase WT to help the Allies mobilize for the war. But if you are planning East versus West, then the Soviet player may not want WT to get high. If WT isn't 100 by the time the Germans initiate Danzig, the US could be facing the problem of being behind in terms of potential mobilization. This occurs in MP games with a human Axis, but now we might be in the unenviable situation of the Soviets wanting to keep WT low to stall the US. If the Soviets forego raising WT and attacking Finland, and Japan never beats China, the USA might find it hard to do everything she wants. Given that Germany is AI, the Soviets should be able to wipe the floor with them faster than the US could get enough assets into Europe.
I'm not even sure what house rules you should use to make the situation fair; once the Soviets occupy Germany, they should have an unassailable advantage in terms of resources and IC.
Strat bombing USSR seems difficult as industry and VP are spread out, still a good idea ? Can US navy do harm USSR by any way ? Won't be Russian tanks invincible for US armies ? Suggestions ?
Let's keep in mind a few things about strategic bombing. The US gets a special national spirit (if she takes the strategic bombing NF) that boosts the range on her bombers by 50%. Unless the Soviets occupy everything from Gibraltar to Tokyo (including Iran, India, and Egypt), you should have plenty of places from which to bomb them with that kind of range.
You should also be able to use that NF to get 1944 strategic bombers sooner than you might otherwise.
And as the US you also get NFs to boost nuclear techs.
There should be no reason you can't have B-29s hitting major points in the Soviet Union with nukes by 1945. You will also need 1944 light fighters to escort them if the Soviet player bothers to remember to defend the airspace of the Motherland.
Aside from nukes, strategic bombers are potent when used properly. You need doctrines researched (strategic destruction, daylight bombing branch), good planes,
and enough bombers hitting the same target to make a difference. 200 bombers isn't going to cut it. But 1500-2000 1944 bombers will obliterate air regions over the course of a few weeks if the enemy can't stop them. And if the Soviets occupy Berlin, that's a lot closer to Britain and easier to bomb. You might as well deny those factories to the Soviets.
Keep in mind that strategic bombing also kills airfields. There are places in the Soviet Union where airfields are sparse. If you kill those airfields, air resistance drops.
But strategic bombing can't win the war itself. It can only neutralize enemy IC or do other nasty things. You will still need to win on the ground. Yes, if there are no restrictions on research, the Soviets will beat you in terms of tanks. Then again, if you can beat him in the skies, you can flood air regions with fighters and CAS/TACs, and grind his divisions to a pulp.
The USN exists to prevent the Soviet Union from getting out of Europe and Asia. You can't really hurt the Soviets with the navy (unless the Soviet player is dumb enough to trade). But the last thing you want are the Soviets conducting Sea Lion.
The problem is that I don't see how the US player is going to stop the Soviet player from occupying Europe with the AI running Germany. The US player would have to be in a position to take Berlin before Germany attacks the Soviet Union. I don't see that happening. And the Soviets have tons of manpower, even if they don't scam manpower from Mass Assault or Mobile Warfare. So, there will be walls of troops covering all ports, coastlines, and possible fronts.
Unless the Soviet player completely fails in the air war, or forgets to put another 3 million men in uniform to cover her coasts, I don't see how the US can prevent the Soviets from seizing enough IC and resources to win by virtue of having more manpower and guns than everyone else combined.