My singleplayer legion for my romani campaign:

Note: max 10 slot for make available a powerful thing... Double legion full stack
I always leave cavalry in one army following main army legion.
When i m in enemy field i leave main army in a fort, and second army (cavalry) around or in one other fort.

Camillan Legion:
1 general
2 astati
2 principi
1 triari
2 leve
1 accensi
1 rorari

Polybian Legion:
1 general
3 astati polibiani
3 principi polibiani
1 triari polibiani
2 velites

Italic Allied Legion:
1 imperator italicus/general
5 fanti pesanti straordinari
4 ausiliari italici (ligures, samnites, bruttian, lucanian)

Marian Legion:
1 general
1 coorte miliaria pesante(aquila)
4 coorti pesanti
2 coorti leggere (ausiliari)
2 artillery or mercenary

Imperial Legion:
1 general
1 coorte miliaria imperiale pesante(aquila)
4 coorti imperiali pesanti
2 coorti leggere
2 arcieri saggitari siriani

Praetorian Legion:
1 general
5 coorti della guardia pretoriana
4 coorti veterane (evocate)