Based on the original thread, I assumed you could take USSR, USA, and China - 1941 planes. Offers some interesting opportunities with the P-40 and I-16 over several games (pilots can hop between those cards without taking hit to Pilot Quality) as they get better each round. Don't feel compelled to stick to just the Russians unless you really want to.
I'm planning to use just Germany, Vichy, and Italy. Plenty of options there.