I'm a fan of HäT Industrie's small-scale figures and soon decided that I needed an army of their Polybian Romans. I mean -- who doesn't need an army or two?
My Polybian legion is currently composed of various HäT Industrie 1/72 plastic scale models. The company is a designer & manufacturer of 1/72 and 1/32 scale soft plastic and 28mm hard plastic military miniatures designed with the plastic toy figure collector, wargamer or hobbyist in mind. I've included notes taken from Wiki regarding the figures depicted.
Hastati (singular: Hastatus) were a class of infantry in the armies of the early Roman Republic who originally fought as spearmen, and later as swordsmen. They were originally some of the poorest men in the legion, and could afford only modest equipment — light armor and a large shield, in their service as the lighter infantry of the legion. Later, the hastati contained the younger men rather than just the poorer, though most men of their age were relatively poor. Their usual position was the first battle line. They fought in a quincunx formation, supported by light troops.
Principes (Singular: Princeps) were spearmen, and later swordsmen, in the armies of the early Roman Republic. They were men in the prime of their lives who were fairly wealthy, and could afford decent equipment. They were the heavier infantry of the legion who carried large shields and wore good quality armour. Their usual position was the second battle line. They fought in quincunx formation, supported by light troops. Principes carried swords, or gladii, instead of spears. Each princeps also carried 2 pila, heavy javelins that bent on impact to prevent them being removed from the victim or thrown back.
Triarii (Singular: Triarius) were one of the early Roman military Manipular legions of the early Roman Republic (509 BC – 107 BC). They were the oldest and among the wealthiest men in the army, and could afford good quality equipment. They wore heavy metal armour and carried large shields, their usual position being the third battle line. During the Camillan era, they fought in a shallow phalanx formation, supported by light troops.
In most battles triarii were not used because the lighter troops usually defeated the enemy before the triarii were committed to the battle. In battle, triarii formed 10 maniples of 60 men each and made up the third line in the legion behind the front line of hastati and the second line of principes. Once the battle commenced, velites would gather at the front and fling javelins to cover the advance of the hastati. If the hastati failed to break the enemy, they would fall back on the principes, who now carried swords rather than spears. If the principes could not break them they would retire behind the triarii, who would then engage the enemy —hence the expression rem ad Triarios redisse, "it has come to the triarii"—signalling an act of desperation.
Velites did not form their own units; a number of them were attached to each maniple of hastati, principes and triarii. They were typically used as a screening force, driving off enemy skirmishers and disrupting enemy formations with javelin fire before retiring behind the lines to allow the heavier armed hastati to attack. They were normally the ones who engaged war elephants and chariots if they were present on the field; their high mobility and ranged weaponry made them much more effective against these enemies than heavy infantry. An early Roman legion contained approximately 1,000 velites.
Pitched battles would begin with the velites gathered at the front. The light troops would fling javelins to cover the advance of the hastati. The hastati were leather-armoured and wore a brass cuirass and brass helmet adorned with three feathers, carried an iron-clad wooden shield, 120 cm (4 ft) tall and a convex rectangle in shape and wielded swords, or gladii, instead of spears. Each hastatus also carried 2 pila, heavy javelins that bent on impact to prevent them being removed from the victim or thrown back. If the hastati failed to break the enemy, they would fall back on the principes, who had also been re-armed with swords. If the principes could not break them, they would retire behind the triarii, who would then engage the enemy.
This order of battle was almost always followed, the battle of the Great Plains and the battle of Zama being among the few notable exceptions. At the Great Plains, Scipio, the Roman general, formed his men up in the usual manner. But once the hastati had begun to engage the enemy, he used his principes and triarii as a flanking force, routing the opposing Carthaginian troops. At Zama, Scipio arranged his men into columns, side by side, with large lanes in between. The opposing Carthaginian elephants were drawn into these lanes where many were killed by velites without inflicting many casualties on the Romans. Once the surviving elephants had been routed, he formed his men into a long line with his triarii and principes in the center and hastati on the flanks, ready to engage the Carthaginian infantry.
Info paraphrased from Wikipedia