It's impossible, as far as I know, to have additional models in a unit, except as 'officer' types, or as 'animal' types (like dogs). However, by 270 BC, the Britons and Gauls were largely not using naked warriors anymore (the Gaesatae being a notable exception, but they acted as mercenaries). If it is possible for the factions to have varying models for agents, I'd not at all object to a 'Druid' diplomat for them, that'd typify the Druidae quite better, though they did fight the Romans at Ynys Mon, but were equipped with the arms of Ordovices, likely due to the rather impromptu usage as combatants.

As for Brennus, yes, off topic, as he was a Gaul, but a truly interesting figure. Also a bit of a testament to the disdain the Gauls had for Romans. CA's depiction of Gauls as filthy barbarians forgets that Gauls, in fact, found Romans to be so filthy that some reportedly would grow angry merely because they were 'dirty'.