I'd not that let that army see the field without re-inforcements of some spearmen, unless it was just taking out Rebels of a known composition.

Terrain, and opponent matter; little things like if on attack or defense. I might do something like :

E Pvvv - Screen Velites, expected to retreat on contact
EPPHV - Block Velites, plan to flank near enemy
G AHH -2 Hastati, + Archers & Velites, create a kill box for the strong unit, that initial contact was refused with, when it advances forward.

Refused right flank. Advance with oblique formation, the frontline Velites run back, meaning the tougher Principes, quickly move to outflank, and the Cavalry can run round the back. With General either re-inforcing the flankers, or aiding the centre/right side as things develop. The opposition weaker troops get rolled up on left, and the cavalry prevent successful withdrawal.

This is an enemy General bagging formation, as they get drawn into the formation, rather than doing the right thing and retreating.

But you're asking a question that there's no set answer to, and about an incomplete unbalanced army, that could be in big trouble and massacred, even meeting a roughly equal force, rather than overwhelming odds. Add the Triarii, unit 2 Barb Mercs plus 1 or 2 Town Watch, and I'd feel much more comfortable.