Gallic Brihentin are the only bodyguards in EB that are owned by the slave faction. That is why all FMs from dead factions revert to them. In previous versions, Carthaginian bodyguards fulfilled this role. In terms of raw settlement count (ie what percentage of the map belongs to which cultures), barbarian heavy cavalry probably make the most sense. The only other general unit that wouldn't look ridiculous most of the time would be the Greek Mercenary Generals.