After Shogun 2 rekindled my interest in TW, I went back to working on a Mod combining Rio's IB BRG world with FOR's aesthetically pleasing troop units.
An aspect I like greatly with FOR's barbarians is thrown javelins as the characteristic weapon. This may be very well in keeping with the age since the references to showers of thrown javelins are so common. And a smart Warrior who injured his opponent with a javelin before closing tended to live longer.
I prefer 60 men as the standard smaller Roman infantry unit for this later period. The legionaries I keep at double strength at 120 men who remain the "heavies" of the force. While the lighter Numerus filling a role the old Auxilia used to.
My preference is for a smaller cohort (numerus) of 300 (actual) men, another element is by this period a portion of the Numerus was lighter troops. Either javelin-men or later more commonly archers. However game wise these have to fight as their own detachments. My belief is at least one "Centurie" of each Numerus was lights in this period. For the Legion double cohort this means two "Centuries".
Later known Byzantine practice had 256 (actual) man Numerus with a third of these being archers.
Having all your infantry as 80 man units I find a trifle dull.
Ice Torque's giant map presents some problems.
Large Map means great distances and heaps of hard coded corruption and loss of funds. Rio overcame this with lots of buildings provide a trade and law boost. For me this was overdone and I removed the trade aspect of things like city walls. I dropped troop costs back to vanilla levels.
I always drop "The base morale level" back to Vanilla levels feeling an army of berserkers was not historically valid.
The basic Roman troop legions, cohorts and maybe basic cavalry I give a metal stat of "3". These romans might well equipped but they can lose without good leadership. I also extend the building period for these basic Roman troops to 4 turns for cavalry and numerus, and 6 turns for the legio.
For surviving comitatus legio with higher morale I use mercenary veteranii which can hired but with Rio's mercenary fort can be retained to full strength. These can be built but I gave it 15 terms.
The trick for me is limiting how fast and how good the Romans can create troops, history refers to difficulty in this regard. Rome even in the 5th century is vast and power, I needed to have more weakness in this regard.