Why do they need to add a new 'soviet' faction? The Soviets fought for the allies. That includes America and the Commonwealth (like from CoH1)
Also as stated in this thread there were a few battle where Japan fought Russia. But the game certainly doesn't have to be 100% historically accurate. Hell, they aren't with the units so why do they have to be with battles? At the moment in custom games you can even go German Vs German or Soviet Vs Soviet (which makes no sense).
Soviets obviously fought for themselves, they never belong to Allies, they just received tons of supplies from them because it was in their business to support enemies of the 3rd Reich. Enemy of my enemy is my friend, that's how it worked.
Also i don't care that Japs fight in very few battles vs USSR in 1938 and 1939, these very not fully escalated conflict between those two factions. And in 1945 Japs army has been destroyed in 12 days