My very basic take on the army composition:
Forward forces: these are located close to possible incursion points, and have various names. In fact, nomenclature is one of the perennial problems of this period because there are multiple names for almost all of the troops.
Central reserve forces: reasonably high status mobile troops that are supposed to move to reinforce the forward troops against a serious threat. Unike the earlier imperial periods, there is a relatively high proportion of mounted troops in these armies.
Troop types are, to the mind of an Early Imperial player, just plain weird. For example, the 'legions' may be called that, but they are hardly the pilum-armed mincing machines of yore. They are also relatively small; you could get away with 2,500 men per legion. A mix of armour types and unarmoured; shields would be large ovals. Weaponry is a mixture of long thrusting spears, quite heavy throwing spears (not the pilum) and, sometimes, heavy darts. Just to make it even more complicated, they could have an integral rear rank of archers and dedicated skirmishers, usually with javelins. Archers could also operate in detached units of their own.
Auxiliae have also changed out of all recognition. Some are unarmoured, while others are pretty much indistinguishable from the legionaries; many elite auxiliae palatinae are in fact of better quality than the legionaries. These guys are at least as regular as the legionaries.
Cavalry can be of excellent quality, there is more of it and some of it is very heavy. You can have the later equivalent of the earlier heavy cavalry with spears, although they look different. You can have cataphracts. And you can have a type wargamers usually call "Clinanarii", which are quite heavily armoured and have lances and bows, possibly with some horse armour. These are tend to be units of the regular army. There are also varieties of regular light horse, armed either with javelins or bows.
The function of auxiliaries is taken by Foederati, which basically means allies, usually tribal. This gives you an excuse to field any 'barbarian' types you like along with your Roman types. Fancy some Visigothic lancers? Not a problem. What about some Huns or Alans? How many can you afford – some contingents will fight for booty.
You can really go to town with an army like this – colour schemes are as wild as the various troops, and there are loads of beautiful figures out there. A quick piccie to finish:
We ran a game of the Catalaunian Fields (aka Chalons) a few years ago, and I still have old blog posts on there about it:
link
There's more under the label of 'Chalons'. I'm no expert, but our lot in Glasgow knew enough to put on a decent sized game. Good luck with the period – colourful it certainly is!
