Since your playing as the Romans, i'll answer with a roman solution
im not familar with history or the exact lines or spacing of a manipular formation

i do try to listen or read what other gifted or knowledgable users say about ancient history of armies and battles n such

and i do remember some stating that the roman machine used mercenaries extensively

anyway

what i do is usually have 2 or 4 lines depending on how large my stack is. Some of my strange methods is to have on open ground battles a full stack of all Rorari (or all Ligurian Infantry)with one general......but to mix things up for you or me if i did different it would be

baring in mind with what ever mercenaries i could get......i would put slingers in the front with spearmen right behind and skirmishers to the left and right of the spear men so they could charge through my slingers ranks to intercept the enemies charge from contacting my slingers.....i have a better idea to convey wtf im trying to type

morale is important in troop selection....having low quality troops and not being careful with keeping your general close by to inspire will result in a immediate rout....especially so fighting the naked fanatics or elephants

example min repeat
(front)Line1:
empty/Slingers/empty/Slingers/empty

Line2:
spearmen/skirmishers/spearmen/spearmen/skirmishers/spearmen

Line3:
Footman/Footman/Footman/Footman/Footman/Footman

Line4:
General and calv
-------------------
-------------------
-------------------
OPTIMIZED less stress example min repeat

(front)Line1:
spearmen/Slingers/spearmen//Slingers/spearmen

Line2:
spearmen/skirmishers/Footman/skirmishers/Footman/skirmishers/spearmen

Line3:
General and calv







hold with a strong infantry unit and flank with weaker also use calv to flank if you have them....the charge bonus is useful