I must admit, that I do not have such an economic view on my armies. I put the best available unit in it and if possible do not mix different unit types in it. Most often I simply put the hero unit, that has spawned the leader, into it, since it can not spawn another leader. But there are still exceptions (z. B. if the hero is a Longbowman and I have Cavalry or Tanks available ). Good thing, that there is no right or wrong way in this game, as long as one has fun and success playing Civ3.