RomanArmyTalk

Hello Gentlemen, 

I became very interested in the armor and helmets of roman commanders (tribunes and higher). 
Why there is an assumption that all roman commanders were wearing an attic helmet and breastplate armor?
What other helmets and armor could they have worn?
I heard somewhere that Scipio was wearing a Thracian helmet. Is that true?

It's because of course of art. Items like the Ludovisi Sarcophagus, etc., which depict such equipment. Whether it actually existed is up for debate.

Early commanders may have indeed dressed in the Greek style. Late Roman commanders are usually mentioned as wearing gilded scale in the text (see Sidonius, Claudian, etc.).