To be honest, I always pretend it’s a KGF game (kill germany first) and go heavy infantry anyway. If it’s Kill Japan First, you can always switch over to tanks and FORCE the Allies into KGF mode, if you want.
But the odds are, you’ll be facing a kill Germany first, in which case, if you are building less then 8 infantry a round, you’re probably screwing something up.
And yes, once England and America get annoying in Europe, you’ll probably lose the ability to target Russian’s specifically. It’s an unfortunate consequence. However, the goal of attacking Russian units instead of the other units is two fold:
1) You are denying Russia those units so they cannot later turn them back and use them to fight off the Japanese.
2) You are preserving your own units and only trading with a nation that earns less income then you.