The way you are suggesting though is use "the French army" (i.e. playermodels, skins, voices) in the Axis slot and just change their uniform skin to a German one and use "the German army" in the Allies slot and change the uniform to Russian. My argument is not saying this wouldn't work, rather it's saying it wouldn't look good. Another way to do it would be to rename all the voice commands, i.e. French -> German and pack them all in the map.
This might work too.
Cool, French SS vs. German communists, I like this idea. No, wait.............I love it.
Where the Germans forced to fight for the Russians or where they actully communist?
If they were mobilized from POWs then probably forced or just lost faith in Germany. Others maybe wanted to rejoin the Germans, hard to tell, I'm sure you can imgagine their motives.