I like the Roman ones, lovely.
For the Greeks, I see it a different way than others as a Greek, but will agree that a too obvious crest might not be too desirable. I love the cape and the plate as an extra touch. Makes them more Greek feeling(and the plate could be transformed to a modern looking protecting vest keeping the general desing).
Have no idea if a modern Athenean is in your plans, but if not, you could stick with the helmet alone, I think it's very distinguishing for Greek units. Especially the front, perfect and the googles great. If you will build Atheneans too, I suppose a very short crest, might work better and look more modern? Can't say I'm a military expert though.
Maybe you could cover the ears in the side, which prolly wouldn't make them that different and would be sensible for modern combat tactics. Also, maybe a bayonet for the gun, though a bit anachronistic, but that's the idea of the unit I guess. I like the idea of adding a xifos, but it prolly change during that time; however I can't offer a solution to what it would evolve, maybe they'd replace it with one of those Gurkha knives?
A suggestion would be an embedded to the suit melee weapon; maybe something claw-like, like Wolverine's claws? Or just a special glove with a pointed object in the middle to stab? Anyway, these are some suggestions; I'm sure you have an idea of were your unit is headed and how will you make it.