Given the limitation of only being allowed 20 units in a 'stack'... What would be the most realistic units to fill up that 'stack' with... What different kinds of infantry and cavalry and importantly how many of this and that? I mean for all of the Roman era's not just early...

Oh and also... What kind of auxilia troops can Rome recruit when they conquer smelly barbarian land? Just plain old lightly armed spearmen or something?