This sounds like heresy, but I'm surprised how similar the Polish and German cavalry uniforms, weapons, and equipment (ammo pouches, gas mask canisters, mess tins) are. You could fudge Polish cavalry from German cavalry with just different heads and sabers. Or vice versa.
EDIT: Even the bayonet scabbard being partially secured by the entrenching tool carrier is very similar to the German method.
Probabilly, given the similarity btw the legs and saddle's equipments, a sort of "universal" conversion kit could be useful, except for germans' smock, but all the european armies need only the upper bodies' part & weapons to build a credible mini. Pratically, a sprue with horse, saddles and lower legs, then you could buy or a sprue with the upper body for the most important armies (such as russians, germans or italians), or an STL file for the "lesser" ones, such as romanians or poles.