The Legion didn't throw the Pilium they threw javelins. Simple light throwing spears with long soft iron points.
-- You might want to recheck whatever source you got that from as the pilum is just a heavy javelin. The romans did use
lighter javelins with greater range though.
-- Worth keeping in mind that the D&D rules model Fantasy Medieval, which already has many of the "romantic" views of combat built-in. This reduces the real-world difference between the way a Legion fought and a medieval host fought (this includes simple things like the use of cadence). Pretty much all the D&D mass-combat rules assume close-order combat with only a few ranks (itself a significant achievement in regards to keeping the troops in order over any distance) and robotic precision with regards to timing (i.e. throwing ranged weapons). All aspects that an ideal Roman army should have.
-- If anything, the D&D rules assume that the Fantasy Medieval soldiers are better at the Roman way of warfare then the actual Romans

Which is fine, since it better models how combat is described in most fantasy literature (or on film, witness the sometimes witless tactics seen in the LotR movies).
The Miniatures Handbook has the Warmaster that, IMO, could easily represent a centurion or optio.
The Complete Warrior has the following feats that would be great for a Roman legionnaire:
Close-Quarters Fighting: You gain an attack of opportunity whenever an enemy attempts to grapple you.
Defensive Strike: If your opponent misses while you are using total defense, you can attack him next turn with a +4 bonus.
Hold the Line: You may make an attack of opportunity against a charging oponent.
Kiai Shout: Shout to make opponents within 30 ft. shaken for 1d6 rounds.
Phalanx Fighting: +1 bonus to AC if you have a heavy shield and light weapon. +3 to AC and +1 to Reflex saves if within 5' of a similarly equipped ally. (Note: Two people can form a D&D Phalanx!).
Power Critical (shortsword): +4 bonus to confirm threat.