There have been volumes written about the Late Imperial Roman army and good material can be found at websites or blogs devoted to this particular period of history. Most of my online research brought me to the classics in the form of e-books, specialist websites all of which are listed with the final installment. Before designing a scenario based on the Germanic migrations through the Gallic provinces we need to inspect a number of key elements; events that took place before the great migration, the administration of the empire and lastly the army.
Looking at information describing details of battle locations and army strengths is very sketchy at best but we do have a picture of how it should have appeared circa 395 AD with the creation of the Notitia Dignitatum. This document not only lists the Imperial civil and military offices throughout the empire but also lists the distribution of military units into border and field armies.
The creation of mobile field armies was first initiated by Diocletian and further developed by Constantine primarily as a means to repel barbarian incursions or to quickly mobilize troops to campaign outside the empire. Five such mobile armies (comitatenses) were formed for the eastern part and two for the west. As our focus in primarily with events in the west we find these two field armies are commanded by the magister equitum and the magister peditum with the latter office based in Italy. The magister peditum also held supreme command over both armies in the west and was subordinate only to the emperor.
Supporting the mobile field armies while on campaign were the limitanei or troops of the border districts. Aside from newly raised units for garrison duty the limitanei incorporated the older legion and auxilia units in their number. Under certain emergencies and these became frequent, we find units of limitanei were withdrawn from the regions to be deployed to the field armies. This became the case of the two field armies lost at Adrianople in 375 AD or during civil strife such as the period of six emperors, each stripping as much of the army strength to make secure their hold on the throne.
To ensure against a period of upheaval as was seen in the 3rd century AD, changes were implemented by Diocletian to separate civil and military offices, such that no governors held military commands. The Notitia Dignitatum nicely illustrates the imperial offices down to provincial level as well as army commands; note also the number of former military titles which now became administrative.
Through the course of the fourth century we find Rome hard pressed to maintain the regular army at strength to fight off the barbarian incursions and attempts by usurpers to take the throne. To assist the regular army on campaign we find allied forces or foederati were employed for this purpose. Germanic tribes, as part of their relocation to lands within the empire, were obliged by treaty (feodus) to render military service within their territory or in some cases deploy with the field army for a period of time (Bury references a term of one year). In the fourth installment (Timeline) we can follow the course of military and political events that brought about a transition in the Roman military.