So why not use the Principes pila if the pila were having little effect on the phalanx? The horsemen were heavily armoured but I doubt the horse would have been, they would be susceptible to the pila, no?
Would the "thurophoroi" be the heavy peltasts of R:TW?

Why would the Triarii have given chase? They were the experienced men of the Legion, they would have better discipline. Another example of poor leadership?