Many Celtic units have javelins and when you turn on autofire, you can repel enemy charge, huge mass of javelines thrown into face could make charging enemy rout.
And the southern units aren't at all bad. My anti-Roman armies consisted of southern swordmen, spearmen (gaeroas, the havy skirmisher, good for the tactic described above), gaesatae, slingers and cavalry and did wonders, with few chevrons they can face Romans.
Also your bodyguards are superior when compared with equites, just charge enemy general and order yours to use swords, they will kill Roman general pretty fast.
The idea not to mess with Sweboz is wise.
And there is one enemy, whose units are even more powerful and harder to kill than Romans, and waits for you in the southwest - you really don't want to fight Cartaginian phalanxes and armoured infantry with your light units.