Although I don't have any example of Roman citizens or subjects serving overseas as mercenaries, there were definitely mercenaries of Italian origin that did. However, you may be interested in the Mamertines.

The 'Mamertines' were mercenaries employed during the end of the Sicilian Wars had seized territory in Sicily. They ruled Messana and the surrounding area for a couple decades. This complicated things and caused a political escalation that would fan the flames of the coming First Punic War. In a rather complex series of political maneuvering, the relatively insignificant band of mercenaries (although in a strategically positioned city) found themselves completely eclipsed by the two Mediterranean powerhouses.

Coming mostly from Greek, Oscan, and Italiote areas of Southern Italy, close contact with Greek settlements led to them to fight mostly in the hoplite fashion.

The Fall of Carthage: The Punic Wars 265-146BC Adrian Goldsworthy