For the pre-Marian units I think the infantry models can stay intact. All the research I've done shows that the vanilla models are reasonably accurate. The only model that needs any real work is the equites one. Equites had capes and chainmail armor with usually oval shields. They were really more heavy cavalry than they were light. Only the most affluent of Romans could become equites cause only they could afford battle-worthy horses.
The rest of the Republican army would look awesome with a quick reskin. I could help with making the Brutii, Scipii and Senate skins after the Julii ones are finished (I'm just blindly assuming that you will skin those first).