The army list for the Middle Imperial Roman army have archers (4Bw) and artillery as options. These are particularly useful when fighting against the elephants or cataphract cavalry of the Sassanid.
The archers are from Old Glory as are the artillery.
As twelve archers are needed for the three commands of Roman the extra bowmen are earmarked for for the Armenians. I plan two commands for the Armenians, so all the archers will be used.