Is it better to have 4 identical divisions in a corps or a mixture like 2 arm 2 mech.
You would usually want to limit the combat width of your divisions to 2 or 3, so that at least four divisions can move to the front line during battle. Besides that, Arm goes best with SpArt and Eng, while Mech is better combined with LArm, AC and SpRArt, all of which will not slow down Mech as Arm does.