Triarii can't form phalanx...

[Tamur]

Note the size difference between the spears of the triarii and those of the Carthginian phalanxes. The Carthaginians used sarissas which varied between 14 and 24 feet long! The Romans went in for the Greek style spears (6-8 feet long).

They could and did use phalankai against the Etruscans and other tribes of north Italy. But forming a Greek style phalanx against a sarissa phalanx is a sure way to lose a unit.

Still, the original point still stands... triarii should be able to.