Happy to be challenged on this, but here's my gues-timate of what a historically accurate Celtic force (Aedui, Arverni, possibly Casse) would look like.

Basically three main groupings: the Elites who are chieftains/magistrates and their bodyguards and nobles; the Warriors who are the professional fighters and mercenaries; the Levies who are the ordinary Celts called to war to serve their chief. My guess is a breakdown where half of the force is Levy, and the remaining half 1:2 Elite:Warrior. So something like a 3/7/10 split of Elite/Warrior/Levy.

So an army might look like this:
Elites
1 Brihentin (ie the general)
0-1 Heavy cavalry
0-2 Heavy infantry (Arjos, Solduros, etc)

Warriors
1-2 Light/Medium Cavalry (Curepos, Leuce Epos, etc)
1-2 Heavy infantry (Neitos, Gaesatae, etc)
2-4 Medium infantry (Botroas, Celto-Hellenic Spearmen, Milnaht, etc)
1-2 Light infantry (Kluddolon, Bagaudas, etc)

Levies
2-4 Missile (Iaosatae, Sotaroas, etc)
Remainder levy spearmen (Batacorii, Gaeroas/Gaelaiche, Lugoae, etc)

Thoughts?