High Imperial Roman army vs Late Roman army

[Frank]

Heres what I know-The Trajan type army was focused on Heavy infantry as its striking force. Those guys had big shields and used the Gladius for stabbing in formation. They carried the Pilum to soften ranks or as a spear against horsemen. They had small detatchment of Auxilia cavalry to guard the flanks and archers from Crete and Syria

This is most propably right for Trajans dacian campaign but could be wrong for his parthian campaign. The roman army of the 2nd century was more flexible than than that. I doubt, that Trajans army at Ctesiphon looked that much different to Julians army 250 years later. Perhaps a few more heavy units with scutum and gladius, because Trajan could not reequip everybody accordingly. But Trajan surely adapted the equipment, composition, formations and tactics of his army to the parthian enemy as much as possible.

You might read Arrians report about the battle against the Alans, just about 20 years later in 135 AD. This was propably the kind of army, Trajan used in Mesopotamia. And this army looked much more like a late roman army, than your standard Hollywood Legion.

The late roman army (pre-Adrianopel/Frigidus) had its weaknesses. But regarding equipment and formations, it was just more flexible. And of course they could fight like an old-style legion with a shieldwall and javelin volley, followed by a short infantry shock attack. But just if facing the appropriate enemy.