The most important source we have today is the Notitia Dignitatum. The problem is that only copies from the medieval exists. But I collected all my study sources to explain what the roman army was in the 4th and 5th century.
I will give examples and sources (books and antique authors). Some books are in fact part of the univercity of Frankfurt/Main when you study old history. Some other sources are mine at home.

Limitaneii:
Were stationed at rivers (named ripenses at early times) or as regular garissons in cities or at limes borders. Also these limes troops were legions! some of these Limitanei were regular imperial legions before.
But decreased in status and equipment after the military reforms.

In the game IBFD one of these garrisons armies should have 2-4 Limitanei (or militia) units. Also 1-2 mounted (cavalry) units (locals or foederati).
Also it is helpful (and historical) to have some minor foederati infantry units like Bowmen (germans or fleet sailors).

Field armies:
The strength of a roman legion in the time frame of Constantius and/or Aetius was 1000-1200 men. According the ancient writer Renatus. that is a fact!
The field armies were divided into 2 parts.
1) the regional field army (comitatenses)
based and stationed at central locations like Gaul, Illyricum, Thrace, Syria, Aegypt. Garrisoned in cities of course.

2) central field army (classed as Palatinii)
later all units (comitatenses and palatinae) became mixted and were put together. But the Palatinii had always a higher status.
But at the beginning you should have only Palatinae units in this central field army. Included regular Palatinae and Auxiliar Palatinae and Scholae cavalry (of course, if you want some sagitarii).
And you should have only 1 central field army!

The majority of the armies foot soldiers were auxilia (strength 500 men). The status of these auxilia forces were (acc. Notitia Dign. and Renatus) Palatinae.
This is a very important information.
So, the Auxilia Palatinae fought most times in the first line of battle (Maybe as shield wall? I thing so).
In this case it is for me absolutely legal to include 2 units of them (better 3!!) into the regional field armies.
Behind the Auxilia Palatinae you can place the regular Comitatenses (4 each, +/- 1 unit).
1-2 sagitarii,
1-2 Exculatores,
if needed put foederatii (like Goths or others) on the flanks.

The Palatinae of the central field army and the Comitatenses of the regional field armies should seperated at the beginning of the game. But later it is absolutely legal to mix them with other units. In the time frame of 410 (like our beloved mod of Ramon) the units were partly mixed. Partly! Not all of them. That's what Renatus said.

The free slots of your army should filled with the most important: the cavalry!
In earlier times (time of Theodosius) the majority of the mounted troops was garrisoned seperatly. Base Camps were in norther Italy, Greece and the balcans. Smaller detechements were always marching with the legions (like equites auxilia and other minors).

The basic were the equites Promotii (you can use 1-2 units). These mounted troops were supported by equites dalamti or equites mauri or equites scutarii. More and more important was the cavalry equipt with bows like equites sagitarii or huns or sarmatians (you can use 2 units mounted bow men).
The cavalry reserve at the time frame of our mod can attack the enemy alone as fast-mobile-task-force. If the time is too short you can atack without any infantry support.
By the way, the super heavy cavalry (like Clibinarii) had the HQ near Milano/Mediolanum. So try to found a garrison there.

The Notitia Dignitatum (and I have a coloured copy at home) teach us that 2/3 (two third) of the army in the east and west were frontier troops. The mounted archers were included to both, field armies and frontier garrisons. The sagitarii infantry archers were (acc. Simon McDowalls book) outnumbered (means less than mounted archers).
The Lanciarii in the time frame of the emperor Constantine III. (the time frame of our mod IBFD) exist only in the eastern part except a single unit stationed with Illyrian Units at the river rhine.

The cavalry in the eastern part of the empire was little different: the fighting elite were the hippo-toxotaii attached to the legions. Supported by light foederatii and sometimes with heavy cataphractii.