Wasn't the degree of "uniformity" even in the post-Augustinian fully professional state-supplied legions still kinda nominal though ? Or rather, functional, especially as armour went - the soldiers got what was available, and insofar as they now had a good set of body armour it really mattered little whether it was scale, mail or segmentata.

In other words, as long as the soldier had all the items of the kit he needed to carry out his purpose (ie. kill the other guys and not get killed), the specific details of the gear (such as the exact type of the helmet, the specific form of the gladius, whatever personal embellishements they now could add to the outfit...) were largely irrelevant.

And the officers pretty much wore whatever the Hell they felt like and could afford. Then there were the various "badges of merit" soldiers of any rank might sport, and apparently also wore into battle...

After all we are talking about a pre-industrial society here. Even if the great arms manufactories were as close to assembly lines as period technology now allowed, the units in practice kitted their troopers out with whatever they had at hand - which might be quite old, looted, modified, repaired many times over, etc., none of which really mattered.