Okay, here are three variations on the classic roman Galea helmet. There are of course many more, but these are the three most prominent ones.
Download:
http://www.filefront.com/14485685/Galea.zip (140 KB

)
They are not textured and are very low poly. Feel free to modify them, just give me credit, please.
Note: I have yet to play your faction but when I do I will tell you which helmet goes to which soldier. For now I just have this:
http://yfrog.com/0yhelmetvariationspGalea w/Plume (Left): This a very basic design of the Galea helmet that has a plume that points backwards. It was used by early-middle Roman Republican armies of Hastati (Low ranking/poor spearmen and later swordsmen), Principes (Mid ranking/moderately wealthy spearmen and later swordsmen), and Triarii. These all were not used after the Reforms of Marian in the 2nd century B.C.E.
Plain Galea (Middle): This a plain Galea, and the norm for later Roman Legionnaires and non commanding troops, such as equites and richer auxiliaries. It was very strong in design and performed marvelously.
Galea w/Sideways Plume (Right): This is a normal Galea helmet with 3 modifications: 1.) the Quality of metal used was slightly higher, as the officer could afford it. 2.) It had bronze or gold gildings on it, making the wearer look "as lustrous as a god's own armament". In other words, it looked good

. 3.) It had a sideways plume that helped him be recognized in the confusion of battle. This was even more helpful when the ordinary soldier stopped wearing plumes himself, as he had no reason to be recognized.
More info to come!
Wait a sec I want to do the fixing of the models tell me whats wrong and Ill fix it ok?
I am not sure what you are asking:
If you are saying you don't want to redo the models, the answer is:
You asked for them, now you at least should consider using them.
If you are asking why they need to be change the answer is:
They are more similar to Italian Barbute helmets, rather than Roman ones.
Edit: Heres some feedback:
General Changes
• Helmet with simple Plume
Spearman (Because he looks like a Princeps spearman)
• Plain
Ballista
Cavalry
Catapult
Guard
Heavy Cavalry
Spearman
Turtle Formation
Wartime Mechanic
• Helmet with Sideways Plume
General
No Helmet/Other Suggestions
Archer should have no helmet
Longbowman: Longbows weren’t invented until the middle ages
Spike Ball Man: How did a medieval era unit get in here AGAIN?!? The Romans didn’t even have flails.
Turtle Formation: How about call it a Tortoise? I don’t really like the feel, or how it has 3800 + Vertices, but since it is a late game unit that’s fine.
Why do all the units have scale armor? That was generally only used by Auxiliaries.
Overall, this faction needs a lot of work, but it is only a beta, so it is expectable. There are other things that could definitely be changed, but I am too busy to do that now.