For Impetvs, the 2nd Punic War mid-Republican Roman legion may comprise,
1x Medium cavalry
1x Heavy foot Hastati
1x Heavy foot Princepes
1x Heavy foot Triarii
1x Skirmisher Velites
Below are two such legions (figures are a mix of Renegade and Aventine):
Four to eight such legions are allowed in a Roman army. I define this as Option 1 where each component is one stand with the Triarii downgraded slightly to account for its historically smaller size. Option 2 would have two stands each of Hastati, Princepes, and Velites in each legion.
Impetvs supplement #4 arrived this week from On Military Matters containing new army lists for Greek hoplites, Alexander and his successors, and Rome and the Mediterranean.
One of the new rules included in the supplement is specific to the Roman Republican army. That rule describes line relief between Hastati and Princepes. Since the Republican army was believed to be capable of line rotation, Impetvs adds this capability as well. Now, given the right circumstances, supporting Princepes can rotate into battle to relieve the Hastati (or vice versa). With this addition, the supporting unit can actually rotate into melee to replace the Hastati. In fact, the Hastati, already in enemy contact, may fight one round of melee before being relieved. The freshly rotated support is then able to fight a second melee with the same enemy. Sounds like a powerful tactic for the Roman player.