The Pahlava are not meant to be a satrapy of the Seleukid's they are the nomads living on the borders of the Arche, apart from a brief time when Antiochos the Great subdued them i don't think they ever accepted Seleukid authority.
Also i don't see how the Pahlava having to take some nomad settlements and wipe out the Saka would be anyway more accurate as that was not the direction they expanded in historically, by 247bc they were in Asaak and by 238bc Hekatompylos, so while yes they do move a bit too fast in the game its more accurate than having them conquer the steppe first.
As for the starting armies the the factions around you are given extra units to provide a bit more of a challenge, even the rebel cities will have larger garrisons. In many cases i expect the team could not know exactly the composition and size of armies at the start date so i guess they're allowed a little leeway.
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