I have never heard of a mod that allow you to have more than 20 units on the field of battle with M2TW.
In EB2 you can give order to a AI reinforcement army but I don't know if this is from Kingdoms or if this is a modded functionnality. I also don't know how reliable it is because I never trust AI to lead my soldiers.
Anyway, back to your main question : to clear the backlines of the enemy in order to charge the rear of the main line you only have to order your cav to... charge.
Skirmishers don't have a chance against mounted troops. Charge, disengage, then charge again til their moral is broken.
When it comes to winning a cav engagement, in my experience the best advantage is gained throught local numerical superiority. The idea is to have one more unit than the AI, or more if your cav is significantly weaker than your enemy's. Engage in 1vs1 fight and use your spare cav to attack from behind. If you can't do this on both wings, focus on one of them and protect the other flank with infantry or cav in reserve.