Thanks for the replies, everyone!

I'll keep an eye out for eastern varieties of archers as my spies explore Seleukia. It figures that the competent archers would be out that way; so far I've only seen toxitai and looked up the Celtic ones since I'm close to the auxilia MIC level to build them in Marseilles. Though I like Watchman's point - one or two Celtic archers would indeed make nice protectors for slingers, in case some cavalry slips by. Such a clever thought that I wonder if that's what the historical Celts actually did with them...