I dont know anything about how the French AI other than it LOVES the Maginot line. I do know however, that you shouldnt have any problem holding the USSR and the Germans (who will be reduced to Poland and have only their Prussian National Provinces, depending on when you stop advancing). It will get progressively easier to hold the line/advance as your puppets/allies you have military control of produce units, and you can probably launch your own offensive into the USSR eventually. Look at it this way: you are ALREADY holding the communists back and actually making gains. Think of how much easier it will get.