Thanks for your replies. An "optimal army composition" is what I am looking for because I had astonishingly little success with my 6 inf, 4 arch Roman legions.
I already realized that a decision for or against archers is problematic.
I hope the game engine is changed so that archers join their comrades in the first line if there is a gap during a battle - so that you do not lose before all units lost their morale.
I mean when a battle begins it is possible that archers stand in the first line anyhow!