Conveyor belt systems are certainly nice. The Scythian mercs aren't that many though. I find preserving them to be somewhat an issue. I personally mix the jav cav and the HA mercs.

Jav cav can take better care of themselves in melee. Also, if you turn off fire-at-will and hand target the javs, you get far better results. Always use them from behind the enemy shields for max damage.

Also, saving a jav for while they charge can enhance the charge in providing additional morale penalties upon the enemy.