That certainly gets tricky, but we need to reconcile 1st century recruiting habits with a 3rd century start date. If the cities of Syria are any indication, Hellenistic citizenry tend to be drawn to a powerful king. The idea that decedents Makedonian military settlers could serve a Pontic king is not an impossibility and likely close to what actually happened in some areas.

I'll have to read those passages though to get a better understanding of the 1st century situation, however. That will probably wait until after exams this week.

The Pontic army defeating the Seleukid one was a combined action with Antiochos Hierax against Seleukos II, correct?

The Pezhetairoi (expect name change for EBII), are the professional military men of the Hellenistic pike corps.