Thanks everyone.
They're intended and based to be used for To the Strongest.
That is a good point and real possibility though I couldn't find clear evidence for them being equipped as such, I'm sure at least some would have had Hellenistic equipment. Polybius mentions they and the Medes broke when charged by the Ptolemaic Greek Mercenaries, so I interpreted it as that they wouldn't have been very well equipped and broke quickly. I'd imagine they just brought what they owned or maybe would not have been worth equipping 10,000 of them from the Seleucid armory, but that's pure speculation on my part.
The main reason I chose against the Hellenistic equipment route is just for variety in the army and having something different to paint other than more Greek stuff. Painting up all these phalangites can drive a man mad!
I could probably get away with using them for other armies outside this specific time period without the Hellenistic gear as well.
Your decision is as good as any! I think the Arabs in Hellenistic equipment is just a wargamer "best guess", although there are hints they were Nabateans.
I wondered if it was TTS - I assume you have seen the free scenario?
Are you doing the Ptolemaics as well? ☺
Neil