I seem to remember that legions used differing equipment and tactics in different time periods. Early Republican armies used three long lines of spearmen, around the start of the Empire these had been replaced by the classic legionary with gladius, scutum, lorica segmenta and three pilums (pila?), and towards the twilight of the empire (with the development of the stirrup and the crisis in Roman manpower) these too had been largely replaced by cavalry. And no doubt there were all sorts of halfway developmental stages between these three extremes. So it's pretty hard to say categorically what 'standard legionaries' carried or how they fought, when any two legionaries could be separated by half a continent and half a millennium of military thought.
I think rkanodia has the right idea, anyway. Some sort of tactical feat like those from the Complete Warrior would seem to be the way to go. At the risk of having this thread banished to House Rules, how about...
Legionary Training
Prerequisites: BAB +3, proficient with medium armour, all martial weapons and shields, Phalanx Fighting.
Benefit:
You gain the use of the following combat manouevres:
Testudo - when fighting using the full defense option with a large shield and shortsword and with similarly equipped allies on either side also using the full defense option, you gain the benefits of cover (+4 to AC, +2 to Reflex saves) in addition to the normal benefits of fighting defensively.
Pilum Training - You may make a ranged Sunder attempt against an opponents shield (only) using a javelin. If this attack hits and succeeds in causing damage after Hardness is taken into account, the javelin has embedded itself in the shield. Your opponent suffers the a penalty to attacks, saves and checks as if he were non-proficient in the use of the shield and an additional penalty of -1 due to the weight of the javelin. This penalty lasts until the shield is discarded or the javelin is removed. Removing a javelin from a shield is a full-round action that provokes an attack of opportunity.
Hardened - you gain a +2 bonus to Fortitude saves made to avoid fatigue from exertion or forced marches.
Advanced Legionary Training
Prerequisites: BAB +6, proficient with medium armour, all martial weapons and shields, Phalanx Fighting, Legionary Training, Weapon Focus (shortsword)
Benefit:
You gain the use of the following combat manoevres:
Close formation - You may enter a square occupied by an ally who is part of a shield wall and who also has the Legionary Training feat. The two of you may fight as normal from that square. You count as being part of the shield wall.
Pilum charge - if you have a javelin in hand at the start of your turn and you use the charge action, you may make an extra ranged attack with your javelin at the start of your charge, draw your shortsword as a free action, and then attack with your shortsword as usual at the end of your charge.
Gladius Expertise - You gain a +1 bonus on all rolls to confirm a critical hit with a shortsword.
Edit: Whoa, in the time it took me to write this, there's been about 10 more replies to this thread! Sorry if I'm repeating someone here...