The rules I use have them rated the same as the rest of the legion. They are supposed to be the older more experienced men so most likely would not be inferior to the rest of the legion. Being older there may not have been as many of them available to fill the ranks. Lifespans back then were not as long as present day lifespans. This might explain why there were only half as many men in each maniple.
The rules I had been looking at grades them as ‘Spear(Superior) whereas most pila armed legionaries are ‘Blades(Ordinary’, which means that the triarii do a lot better against cavalry. By the time of the army described by Polybios, the hastati and principes used pila and sword, not the thrusting spears the triarii at that time. Grading triarii the same as the hastati and princepes does seem to solve a couple of possible wargaming problems.
Presumably, in Spain, where the army seemed to go over to using cohorts rather than maniples as the basic tactical unit, a cohort would consist of a maniple of hastati, one of principes and one of triarii. It is difficult to imagine the different components of the cohort being composed or armed differently.
Your point about the maniples of triarii being half the size of the hastati and princepes maniples is as Polybios describes. However, Livy (who, it is to be admitted, wrote several centuries later than the army he described) said that the earlier version of the manipular army had similar numbers of hastati, princepes and triarii. He said that only the hastati used pila at that time; both the princepes and the triarii used thrusting spears.