The Caesar3 game builders obviously mixed and matched enemy appearances, formation types, and unit strengths freely between different armies.  This seems to produce historically accurate barbarian armies and gives enough variation to make military adventures more unpredictable.  The index below will link you to various studies and help you make the selection  and size specification of enemy armies easier.
All studies were done on open, flat, terrain.  Obviously, a real conflict will not happen in these ideal conditions, but you can get everyone in the team picture this way.  Evaluation was subjective.  Ranks of 0 - 10 where 0 is no effect at all on Caesar's force and 10 is a decisive defeat.