Still on the topic of Roman armies, I believe they used a 2:2:2:1 numerical ratio for their core troops (Hastati:Principes:Velites:Triarii), with roughly equal numbers of allied/auxillary/mercenary troops to cover the flanks. So, in game terms, this could be 2 Hastati, 2 Principes, 2 Velites (or Roman Slingers, I forget the EB name for them), 1 Triarii and 7 Auxillia. These would be led by a general (usually >30 years of age) and with a cavalry wing led by a younger tribune (general aged ~20-30).

In my RTR games, this type of army tends to do pretty well. The numbers appear pretty low, but that's the best way to represent the Legion with a single stack. You could ditch the Auxillia, but they're usually not too bad anyway and can normally hold their own. The only weakness is against cavalry, even when 4/7 of my Auxillia forces are made up of spearmen. Even the lightest cavalry charging through the Hastati hurt like nothing else...

Hope that helps.