On the left we have a century of Auxiliaries, with Hamian Archers behind. The auxiliaries are armed with spears and oval shields, and I consider them to be weaker troops than the Legionnaires, used for secondary duties (e.g. guarding the baggage train or siege engines).
To their right is the auxiliary cavalry, with a line up of spare officers in front. I haven't decided quite where these guys are going yet, but the two with yellow crests & cloaks will be in charge of the siege engines. These rest are wearing a selection of spare helmet crests.
To the right of the cavalry I have a 10 man unit of Parthian Archers (thanks to Tim for nation identification and Gepetto for sending the figures). I'm planning on giving these guys gold scimitars to replace the daggers when I can find them. I don't know if this is accurate, but I like the look. I'm unsure if foreign Auxiliary units would have Roman-born Centurions (and Signifers and Cornicerns) or would supply their own, so for now I have a Roman Centurion in charge, and Parthians as Cornicern and Signifer. All my cornicerns will eventually have wolf head-dresses, and the signifers lion head-dresses.
Behind them with red crests, a specialist 10 man unit of... what? I see them as either mountain troops (wearing cold-weather uniform, hence the trousers and boots) or Pioneers/Engineers of some sort. Possibly they could guard and operate the heavy siege engines. I really like these figures (I didn't know until yesterday that the body is printed both front & back), and want them to be an elite unit, hence the red crests and cloaks, to mirror the Praetorian blue crests and cloaks.
These green shields I find a bit odd - having the handle on the right means that the klicky cannot hold it in front of him facing forwards, instead it protects his left side. The red rectangular Roman shields I flip over and have the handle on the left, so the shield protects the front facing forwards, but that doesn't really work with these guys.
The Praetorians you all know. In some images (e.g. the film Gladiator) they have black as their primary uniform colour, but that appears to be pure speculation, and I've also seen Praetorian re-enactors in red. I'd assume the re-enactors know what they're doing, probably more than Hollywood researchers. I like the blue, and they make a good bodyguard unit with their spears. I need to read up more about Praetorians, but I concur with the theory that spears would be more useful than pila, because they are better for forming a bristling defensive shield wall around their leaders. A pilum is designed to be thrown, not held and thust with, as Gordon mentioned earlier. It's hard to tell what was carried from carvings and statuary, because of artistic license.
Behind the Praetorians I have the Hamian Archers who gave up their kit to the Parthians.
It seemed a shame to waste these figures when I have enough crested helmets, shields and pila to form a new unit. What could they be? I'm leaning towards having these mixed in with the Hamians to give better defense, much as units at the time of the English Civil War would mix pikemen alongside musketmen. While the musketeers reloaded, the pikemen would defend them against sudden attack such as a cavalry charge. Did the Romans do this? There's no reason they couldn't, but I just don't know right now. I'm awaiting a book that deals with Roman military tactics in greater detail, but even that is very much speculation based on equipment.
Behind the Hamian footsoldiers stand the Tribunes.