Hello there.

Your question isn't easy to answer because of the circumstances of the time period and the great upheaval that was to follow it. The fact is, we simply don't know as much about the Roman army of this period, let alone the 'germanic successor states' and their militaries. The answer I will give you is more complex than 'yes' or 'no'.

Over it's long history the Roman Army itself simply became more 'barbaric'. This has precedents deep in Roman history - from Augustus' German bodyguards to the well known preference of recruiting among 'warlike people' such as the Illyrians, as opposed to the Italians. Even Arminius, the German who led Augustus' armies into the disastrous ambush at Teutoberg Forest was a Roman officer with Roman training. By the end of the Western Empire this recruiting preference had shifted strongly to the Germanic peoples.

Even in terms of Total War: Attila: When Stilicho campaigned on the Rhine circa 400 AD he came back with an army swollen with Germanic recruits because there was no other source of manpower. Alaric, a contemporary of Stilicho, was a Gothic commander who essentially extorted the rank of Magister Militum out of the Emperor in exchange for his services, thereby giving him and the Gothic people access to Roman armories, and Roman military training. Alaric would go on to sack Rome itself, and both men are featured in Total War: Attila and Stilicho himself was half-Vandal.

So by the end of it all, 'yes' the Germanic successor states had militaries that resembled the Western Roman army at least in the near-term. Some of the men in these armies would have inevitably served in the actual Roman army, or been sons of men who had. Their commanders may have had direct military experience in the service of Rome. Certainly their weapons and armor would have been typical of the Roman legions of just a few decades earlier.

But again, I cannot emphasize enough how this was a two way street. The Roman Legions of this day had long since abandoned the short, stabbing sword of the Gladius, and the rectangular shield. Germanic war cries and war customs such as the barritus were prevalent in the army by now, and the days of an army composed of Italian, citizen-soldiers were long, long gone. So it's important to understand that the Roman Legion and the Germanic Warband had been steadily converging for over a hundred years by this point, and by the time the Western Roman Empire fell both resembled one another, and the Germanic kingdoms carried on some of this in the short-term.

But in the long-term, the Germanic kingdoms came to resemble a very proto-feudalism structure and they never reached the level of organization that the Roman Legion had, or maintained a standing army as the Romans would have understood it. The Visigoths in particular had a military structure wherein individual nobles were responsible for raising X number of men (by memory, it intervals of 100) that could then be fit together in an organization that did resemble that of the Roman Military, but again this has feudal elements as well because these soldiers weren't a standing army but instead raised from a local lord's holdings at a time of war.

So at the end I'll conclude by saying that I think Total War: Attila represents the germanic militaries 'well' but not perfectly. Probably as best as they could have been expected to.

For further reading on how the Roman army actually functioned in the later stages of the Roman Empire, its equipment, tactics, and relationship to the Barbarians I would reference

Stilicho: The Vandal Who Saved Rome by Ian Hughes

Aetius: Attila's Nemesis by Ian Hughes