You could depict the Romans that fought Pyrrhus or the broadly similar, slightly later Romans that fought the Macedonians and Seleucids. Although there are some differences of detail between the two (No shields for the velites, for example), they will be broadly similar to the classic 4 line (Velites, hastati, principes, triarii) force that fought Hannibal. Definitely post hoplite.
I can't guide you on 15s, though, I'm afraid; it's strictly 28mm chez moi.
Simon