I'm not 100% on this but I don't think Tigranes used a Hellenistic style force at Tigranocerta. The army was centered more around his force of cataphracts, 15,000 strong supposedly. I think that it was more of a traditional Persian-style, allowing various soldiers to fight in their traditional style with a crack cavalry force.

Mithradate's early armies were certainly phalanx-based and he learned the limitations of such forces during early defeats against Sulla. As everyone else mentions, later Pontic armies were based around mobile infantry.

And yes, numbers are inflated heavily.