Use of Cataphracts

[Copperhaired Berserker!]

Since noone else has answered, I'll give it a go:

The proper way to use Holosideroi/Cataphracts would be to use them as the "hammer", when the enemy is already engaged with the "anvil": Your infantry.

A quite well-known Macedonian, I believe his last name was "the Great" or something like that, used this tactic with reasonable success with his Hetairoi/Companion cavalry (hammer) and phalangitai (anvil).

My goodness, you don't know about Alexander the great? Traitor! get him!

[Keyser]

Cataphracts rely on their armors (additional protection) and weight (momentum) to crash directly into the ennemy, they aren't the most suited for running behind ennemy lines and chasing opposing cavalry...
So i would say that using them like Alexander Hetairoi would be a bad way to use them.

I don't know how the seleucids used them but a charge while in a very dense formation (to prevent the horse to try to evade the shock and focalize the momentum) is the only way i think. Of course, you still can't charge pikes frontally :)

They might be vulnerable to harrassing by horse archers (despite the armour) so it's maybe the reason why some cataphracts units also used the bow.