Then why do you keep talking about "realism" in your posts? It is fine if you think it is immersion breaking but that's separate from what is realistic or historical.This isnt about rome at all or a history lesson.
Anyway, the only way to keep defector lords using primarily their native culture troops is auto-regenerating troops like minor factions do. Otherwise, they are restricted to what they can recruit from towns and villages, like the player. So if a Sturgian lord leaves Sturgia and works for the Aserai as emir of Askar, then he's probably going to be rocking a lot of Aserai troops and not much else.