actually the early romans did use the phalanx. they adopted the phalanx after they clashed with etruscan hoplites. infact it was the roman phalanx that defeated the etruscans. and of course the etruscans most likely adopted the phalanx from the greek colonies in italy. hoplon sheilds, bronze helmets and cuirass's are found scattered through out italy. the accounts of livy and dionysius suggest that the servian reform (578-534 bc) converted the roman army into sort of a hoplite army. im sure you would find alot of this in the history section of this site. then, somewhere in the late 3rd or early 2nd century they adopted they started using the iberian gladius along with the heavy pilum javelin. at this point the hastati and princepes carried two pilum javelins and the gladius as the triarii stuck with the hasta (hoplite spear about 8ft in length). so the romans did use the phalanx for quite some time but converted their weaponry, formations, and armour to become more flexible and efficiant on the battle field. their are many posts on this forum that give precise info on what periods the romans started using certain weapons and armour and why they did. just a couple pages back i think.
ive always thought combining the macedonian phalanx with the legion would make you damn near invincible. it would be impossible to give have the phalangites to carry the scutum and fight effectivly. the sheild and armour of the phalangite was less because it was compensated with the long sarrisa pike. the idea was to keep the enemy at a distance yet run right over them at same time. sort of like "i can get you, but you cant get me ". the legion would work great in either holding the flanks of the phalanx while the calvary swings around the battleline to hit the enemy's rear or use the flanks as a key position to enveloe the enemy while the phalanx smashes through their center. or you can have the legion up front and the phalanx in reserve like the trarii. or vice versa to put the legion in a more flexible position to deliver a decisive blow.
please correct me if i said anything stupid. its way past my bedtime