Ive found that Medium or even Light Cavalry are more than adequate for causing routs when charging from the rear against engaged troops. Ok, so Im playing as Baktria... but still with almost any faction the impact is enough. The key is not to come in too early IMO. Allow the enemy troops time to properly engage your infantry (defensive troops - aka spearmen - are best used in this role) then charge the rear, withdraw immediately, charge, withdraw etc. Usually against tired troops 2 or 3 charges are enough and casualties tend to be minimal if you pull your cav away fast enough.

Things to look out for that lessen the impact of the charge include:
Charging through another (friendly) unit
Not having the entire unit far enough away and stationary before the charge order is issued
The enemy unit has changed its facing and is no longer showing its back to you
Enemies are fresh (very unusual with this AI...)

etc etc