The Polish II. Corps in Italy and the 1. Armoured Division in France vetted all German prisoners they captured - and often those captured by their allies. As the war wore on, the Germans grew increasingly lax in who they considered Volksdeutsche (ie Germans outside Germany). Those that registered as Volksdeutsche enjoyed greater rations and economic freedom in Poland and many who had only one German parent registered and were accepted. Later in the war, living in what had been Germany 1914 and speaking passable German could be enough to be registered, regardless if you wanted it or not.

Volksdeutsche were eligeble for conscription in the Heer, the German army, and the Poles in Italy and France spoke Polish to all German prisoners. Those that could reply in Polish were usually offered to switch sides and join the Polish forces in Exile. From late 1943 onwards this was the primary way to replace casualties for the Polish forces in exile.

As for other prisoners of war, I have not heard of the Polish forces in exile deviating from the legal treatment of prisoners in any systematic way (revenge killings and murders was conducted by individual soldiers in all armies).

Soldiers captured by the Polish army in the 1939 campaign were freed by the Germans - the few Poles that managed to flee to Romania in September-October 1939 did not bring any German prisoners with them as far as I know.