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Is there anything that can be done with Fireforge's plastic Byzantines? I know they are a bit early but I don't think there is a range for later Byzantine forces.
As for style, my reading indicated that the Byzantines were quite "archaic" in dress, right up to the 15th century. The wikipedia article claims they were more advanced than their western counterparts. I'm not so sure, the traditional view is that the army was destroyed at Mantzikert and the Civil Wars that followed, and then had to be supplemented by mercenaries who sometimes outnumbered the natives.
I think you really need to focus on a period !