Where was the Roman Army in AD408?

[Nathan Ross]

Constantius III had no federate troops except maybe a handful of Alans

How do we know what sort of troops he had, or did not have? Does any source give either the numbers or composition of his force?

(presumably he still had those ten thousand Huns though! )

Actually, does any source refer to an identifiable Roman military unit in the west after c.400? (the five tagmata from Dalmatia excepted...)

there was a strong military presence in Italy until at least 444.

How do we know this, as well? Is there anything to suggest that the various 'Roman' armies of the 5th century were not just amalgams of foederati and mercenaries?