Historical reference would be Massilians and Galatians, who had 'Hellenized' or 'Celticized' warriors, who adopted various bits and pieces of the other culture's society. Very interesting mixture of things, at that, and we thought it'd be a nice touch for Celts invading Greece, or Hellenes in Celtic territories, to also perhaps get similar units, representing the integration of some locals who willingly embrace some aspects of the foreign culture.