I like "national" troops to wear national items of dress for at least a few figures in the package. The Free French troops picked up 1940 French items of kit after D-Day and wore them. They were fully equipped with US Army gear but being French wanted to show they were French. So many obtained Adrian helmets and other bits of gear. I suspect the Polish troops serving with the Soviet Union did the same as they went through Poland.
As for wearing hats in combat, sometimes there is a shortage of helmets. Sometimes troops are caught in rear areas and don't have time to switch from hat to helmet. Sometimes, particularly on a night patrol, noise is important. So troops will wear a cap rather than a helmet, because helmets make loud noises when they scrape a bush or tree limb.