Wonderful post.
Actually in EB i use camillian principes on the first line, with hastati on the second for supporting with pila and attack flanks or behind of units engaged by principes. This because hastati are lightly armored and use gladius for secondary weapon, having a greater kill ratio than principes. Spears of principes also prevent my first line to be charged by cavalry, while hastati could have much more casualties instead, and the same is for enemy missile weapons vs. principes better armour. Triarii remains on third line, ready to enter in battle for hold the line with their "classical hoplite shield wall ability" (using BI_exe ). This tactic associated with a quincux battle formation allows me to destroy entire phalanx-based greek, epeirotic or punic armies reporting few losses.
After the polibian reform, the better armed and armoured hastati gain finally the first acies, for weaken and tire the enemy before principes engages.

I think that EB reform & unit system represents well the changes of roman army in 3rd century BC.
The Camillian era shows the manipular warfare adopted in samnitic wars, still with great dinstinctions between different classes of roman society and experience.
The Polibyan reform shows the final stage of the process, with a role-consolidated three battle line army where equipment and census makes much less difference than past times; this uniformation permits more versatility and allows to the republic to deploy more numerous armies to face successfully new enemies outside the peninsula, like carthaginians, macedons, greeks, celtic and iberian tribes.

This goes until Marius' times, where the lack of recruitable men forced to abolish census requisites for create a true professional army. Marius ends a cycle started 230 years before, with the roman defeat at the forcae caudinae in 321 B.C. by Samnites. In that circumstance the senate reformed for the first time the army: the legions passed from 2 to 4 and every legion was divided in 30 maniples of 120 milites each one. With this new manipular army, the Res publica conquered definitively Italy, destroying an italic-coalition army on the river Sentinus in Umbria, in 295 B.C. (italic casualties were reported as 100.000 for contemporary Durides of Samo, 25.000 for Livius.) Five years later, Samnites surrendered and the roman victory at Maleventum in 275 B.C. get Pirrhos definitively out of western affairs, leaving Tarentum to its destiny in 272.

Plebeian fights for civil rights during all the republican period contributed also to army reforms, gradually eliminating differences in equipment and warfare. Masses of citizens-infantrymen needed always a better equipment, as like as their political weight grew up. With the Leges Liciniae Sextiae in 367 B.C. the patricii cannot continue to take all the ager publicus for them, and one of the two consuls must be of plebeian origins. In 287 B.C. plebiscites assume lex valour; in 300 B.C. plebeians obtain the right of access to the last public charge denied, the pontifex maximus. The process goes on, till Tiberius and Caius Gracchus times and the successive division between the "boni" optimates and the populares, also in the senate.

Bored? Yes, thanks. Oh, and sorry for my ainghlish

BTW: The names of the three lines of soldiers, hastati, principes et triarii (or Pilani ?) comes from the pre-manipular formation: Principes, the principal force of the legion, the best men, formed the first line; Hastati, younger and poorer, armed with an hasta, then passed to the first line with their pila; Pilani or Triarii, maybe armed with pila in ancient times, then armed with hasta in polibian time.
That's for Arnold Toynbee in "Hannibal's Heritage".