Its definitely quite viable but I prefer to use archers or slingers (the only real skirmishers mainland Celts get) in the front where the enemy will usually either charge forward in anger or send cavalry forward in anger to "disperse" my troops. Keep skirmish mode on and that inevitably leads cavalry into the meat grinder that is my main line. Especially good for getting the really heavy cav to engage your really heavy infantry, especially considering that the heaviest cav a Celt gets is Brihentin, which would only make a solid medium cavalry in the cav-friendly factions (Hellenic and Eastern).