Idea

[Ludens]

well lorica segmenta is the only type of roman armour i could remember and it sounds cleverer. but if it was an armour upgrade it wouldnt take up a unit slot thingy would it ?

True, however armour upgrades are problematic for another reason (one cannot adjust the exact magnitude of the upgrade). Also, technology spread through other ways than conquest. For example, the Macedonians and Greeks adopted chainmail despite never conquering a Roman or Celtic town. And, to get back to your example, what would the Gauls adopt if they conquered a Roman town? Chainmail? Celts invented chainmail. Roman shields? The oval legionary shield was a direct copy of a Celtic design. Romans helmets? See previous answer.