Given the scale of the game, the system in To the Strongest works very well. Line exchange is a special type of rally activation that cane be done when near the enemy. Removes disorder (and refreshes pila if replacing hastati with principles). I model the unit the same way as all deep units, 3 ranks deep, so you literally swap the ranks and it is obvious when you reach the triarii. The original system in the rules used lots of small one hit units, and that also worked fine. You could stack two of the units in a square and swap them to do line exchange.