The real reason I like to believe the Imperials are based more on Romans is that they based much of their culture on the Greeks. The Imperials based much of their culture on the Aylieds from what I gathered. And I recall reading somewhere that Colovians were deliberately based on Spartan like culture, where the lower half of the empire was based more upon standard Greco-Roman cultures. It's not as apparent in the game because for every in-game race there's 5-6 different cultures you could saw they have a basis on.
Here, this should answer your question better than I have.