I am wholly for "geographical" rather than "historical" approach - after all this all about role-playing and game-playing. The legionary names shoud therefore help the player to keep track and give him more immersion rather than confuse him.
I think it woud be useful to think of the legionary banners (ANCILLARIES) - available from the very beginning of the game - as distinct from the "legionary_name" attribute, which shall be limited to Marian and Augustan era first cohorts (and which might bear proper "historical/geographical" names if you wish).