I don't find it strange at all. They're pirates, after all, and would be a motley crew anyway. I like the Roman gear especially, since the Romans were a big target for them and would be subject to piracy.
I know what you mean, though, about the lack of information. I'm doing a couple of 15mm Pontic units at the moment and it's the same. There really isn't much information on how they looked. I'm using designs from coins for shields and banners, perhaps a Total War: Rome II design or two. Lot's of mauve, turquoise, sea blues, wine-stain red for my pallet. I just painted the plumes on the scythed-chariots' horses as peacock feathers - just my own speculation as one: when my wife and I took a vacation to Side, there were peacocks roaming freely on the hotel grounds and I figured they were "eastern" enough for Mith and his Persian influence. And two: just got tired of painting the same old red, white, black, and purple plumes.
But there's really no information, just my own speculation that I hope is in the ball-park. So your pirates look like pirates to me, and they look really good, too.