I'm no historian either but I get the impression from playing EB that it was rare for a state to export it's fighting style to far-away conquered lands, usually they just recruited from the local culture. The Romans were one of the exceptions, so it does make sense for them to recruit legionaries in more provinces, I'm interested to see if their recruitment will change in EB II. As far as the Saka Rauka are concerned, remember that the Saka hoplites wear Saka style armor, and I doubt you would find many Saka armor smiths in Byzantion.