Armored in leather torso armour and carrying stout shields, they are well able to last in melee.
Hail Stratege! The men mounted before you are among your most vital cavalrymen. Armored in leather torso armour and carrying stout shields, they are well able to last in melee. After throwing their iron tipped javelins, they will retreat behind the shock cavalry, and then help to exploit the breakthrough made by the lancers by weight and skill. Do not misuse them, for they are an elite among your horsemen, and are difficult to replace. Still, they are well able to hold their own in virtually any melee, since each man's heavy kopis will deal death to those infantry who dare approach them.
Historically, the successor states fielded cavalry different from what Alexandros had used. These men solved a very practical problem: how do you soften up an infantry line for a charge, and then exploit it afterward? After the early days of the successor kingdoms, there were never enough men of this stature to make a massive impact on the battlefield. But, perhaps you will learn from the mistake of allowing your own opulence overshadow the needs of your army?