I have been considering getting a small WW2 Polish force in 6mm recently. I have made a few notes on my blog, which might be helpful to others; on the other hand, people more knowledgeable than me might know of other ranges which have escaped me.
You say French infantry can be used at a push. For 6mm I am sure it would work.
Yes, AFAIK many Polish infantrymen (the mobilized reserves) still wore the French ‘Adrian’ style helmet – since hats and weapon outlines are probably the most distinctive items on 6mm troops, then it should work okay.