The difference between hastati and principes is one of age (and the experience it brings), and nothing else. The representation of heavier armed principes and lighter equipped hastati is something of a convention to show the variety of armour available and uses the assumption that older men have accumulated more wealth. In history, nothing would have prevented a father giving his son his chain-mail as he left for his first campaign among the hastati.

Montelfortino helmets are not the only helmets. Just a common type. This is not a standardised army we are talking here, but a citizen militia.

The rorarii are poorly understood. Our surviving written sources are all centuries later (Livy, mainly) and may be talking about more than one type of soldier. At one point they are skirmishers and at another can be confused by the enemy for triarii.