Well, the first thing I would want to do is narrow my focus just a bit. After all, the Achaemenid Empire lasted for 220 years, and the army of the Great King did not remain static during that time. Are you looking to represent the forces of Cyrus the Great, Xerxes, Artaxerxes II, or Darius III?
Or by ‘viable,’ are you perchance referring to some sort of generalised, points-based, tournament-oriented thingie?
"Ventosa viri restabit." ~ Harry Field