I've been playing it for a while now but I didn't realise that you could have the AI control your forces.

Do you have cavalry only as your flank and infantry as your centre or do your flanks contain a mixture?

With larger formations I've generally gone for:-

H | T | T | T | H
E | H | P | P | P | H | E
G | V | A | A | A | V | C

where:-
H=Hastati
P=Principes
V=Velates
T= Triarii
E= Equites
C=Cavalry
G=General