The Galatians indeed started fighting in the roman fashion, as they were trained by roman officers and fought under roman commanders like Pompeius Magnus and Caesar. Eventually the galatian military was formally formed into a legion. However, at that point they historically were not much more than a roman Type IV government, in EB terms.

I remember reading some good arguments against their inclusion as an own faction, though I would have to search for the respective thread, and I myself woudn't mind seeing - and playing - them. It has something to it leading a celtic army in an environment like that.