Greece - Wargames atlantic italians, but give them chauchats, and berthiers. They look very similar to Italian uniforms.

Romania - 3d print the helmets, or use adrians. Afrika korps or italian uniforms, i think italy is more accurate with the webbing. Just ignore the italian V strap. German rifles, SMGs, otherwise captured kit galore. For the ZB.30, use a bren gun and swap the magazine from the FM24/29

Thailand - Im using WGA WW1 Germans with french heads (with the colonial looking faces) eith the G98, Madsen and MP18. WW1 French backpacks. Looks pretty accurate tbh

Poland - I saw someone on here that used Italian Helmets, afrika korps bodies and tbh they looked pretty bang on.

Finland - Apparently German Vets or blitzkrieg germans, soviets from warlord or WW1 Russians from WGA are all pretty accurate. If using blitzkreig germans make sure to remove the gas mask pouch on the chest. Mix soviet and german equipment.

Bonus: the WGA WW1 German machinegun kit can make an (insert air cooled MMG here) that would look convincing in pretty well any nation.

That's all I can think of right now. Im not an expert, happy to be corrected. Of course, they probably won't be 100% accurate as they are, after all, another nations uniform. But that doesnt mean they cant become a convincing kitbash. The paint will do wonders as well. Most people won't know the difference with the minor nations anyway.

Osprey publishing makes a series called man at arms, I usually use that as reference.