Actually most countries met at least on limited scales this is mainly because of the battles on the balkans. Both sides reinforced their allies. This resulted in the prensence of pretty much all armies. Interestingly its more likely that allied countries never fought in a battle together than that they didn't met all central powers in combat. Keep in mind only 3 countries fought along side Germany.

I already started a write up where all armies met but this would certainly be too much.

Did the Italians and Germans ever meet in combat?

Yes. Austria-Hungary had trouble breaking the costly stalemates at isonzo and requested support from Germany. Germany send several divisions which had a major impact in the following battles. With the help of these Divisions and German leadership the stalemate was broken and the italian army suffered heavy defeats which prompted GB France and the US to send reinforcements.

Did the French or Americans ever engage the Austro-Hungarians or Bulgarians?

French troops fought K.u.K at the italian front and in the balkans. The US had troops in italy and engaged the K.u.K but i doubt it had contact with ottoman or bulgarian troops.

Who didn't met.

The US forces only met Germany and Austria-Hungary.

I doubt Japan met bulgarian or ottoman troops. But it engaged German troops in asia. K.u.K troops were involved at least naval.

Its quite possible that greece troops didn't actually met german or ottoman on the battlefield but iam not entirely sure about this. They participated in the same battles but with limited numbers which makes it possible that they didn't met.

Portugese troops were likely only fighting german troops on the western front. If they met other middle powers it was likely very minor or a naval engagement.