How close to a realistic Seleukid army would this be?

[marodeur]

I understand the Seleukids suffered some spectacular defeats at the hands of the Pahlava...

That's true. And with your army composition you will suffer some quite historic defeats against historically correct pahlava armies consisting primarily of horse archers and better-than-you-will-ever-have cavalry units (which you can't ecpect ai to generate by itself). But your army will diw in historically correct fashion... .