Where was the Roman Army in AD408?

[Flavivs Aetivs]

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?

Well the laws of the 440's indicate army units were being redistributed to fortified towns under the command of Sigisvult due to the Vandal invasion. Aetius had at least two field armies prior to that, because we see him operating independently of both Felix in 426-430 and Litorius in 435-437. He had the forces to campaign against the Franks while Felix was campaigning against the Huns in 427, and while Sigisvult was leading forces against Bonifacius in Africa (427-429).

We know some sort of force was operating in Spain as well, under Censorius, Asturius, and Merobaudes although it was destroyed probably under Vitus in 446. When the Romans and Huns were defeated at Tolouse in 439, Aetius came out of Italy and defeated the Goths with some sort of military force that same year, ending the war with a Roman victory.

I can't go into detail regarding Constantius' III's available forces without some research. Honestly it's been a while since I've discussed this.