Well if you want it accurate, you should use a few Triarii, followed by a bit more Principes and a whole lot of Hastatii with a bit of cavalary.

The realistic army composition (and tactics) before the Marian reform would be pretty bad actually since the game limits you to fielding relatively few units and you would be outflanked by the enemy all the time.

Realistically, the Republic-era roman legion consisted of roughly 1200 velites, hastati and principes each, 600 triarii and 300 equites. Each legion was usually accompanied by another army, roughly the same size, of the socii (comprised of italians under Roman influence who did not have the Roman citizenship). It is assumed these socii were organised in a similar manner than the roman legions but also brought troops to the battlefield that the Romans were lacking in (mostly cavalry).

After the Marian reforms and then the social wars these socii were integrated into the Roman legion as first the socii soldiers and then the entire italian penninsula were granted Roman citizenship. Legionaries were now full-time professional soldiers and would be the main body of infantry, a bit over 5000 per legion with ~500 roman cavalry. To replenish their numbers outside of Italia they would hire auxilliary troops, either local militia, trained troops from the region or mercenaries to fill in the role of cavalry, light infantry and skirmishers. Especially after long campaigns the auxiliaries would often outnumber the legionaries.

Late reply, but perhaps someone else can benefit if wanting to play historically.

Camillian/Polybian Army Composition:

4 units mixed skirmishers/slingers (2 roman, 2 auxilia)
4 units of light infantry (2 hastati, 2 auxilia)
4 units of heavy infantry (2 principes, 2 auxilia)
4 units of spearmen (2 triarii, 2 auxilia)
1 General
1 Equites
1 Auxilia Cavalry
1 Mercenary/Artillery/Specialist/Auxilia

From the old Europa Barbarorum days:

https://forums.totalwar.org/vb/showthread.php/101787-Quintus-Sertorius-Guide-to-Conduct-Becoming-of-a-True-Roman-%28Redux-for-EB%29

Originally posted by CoyoteJohn:

Late reply, but perhaps someone else can benefit if wanting to play historically.

Camillian/Polybian Army Composition:

4 units mixed skirmishers/slingers (2 roman, 2 auxilia)
4 units of light infantry (2 hastati, 2 auxilia)
4 units of heavy infantry (2 principes, 2 auxilia)
4 units of spearmen (2 triarii, 2 auxilia)
1 General
1 Equites
1 Auxilia Cavalry
1 Mercenary/Artillery/Specialist/Auxilia

From the old Europa Barbarorum days:

https://forums.totalwar.org/vb/showthread.php/101787-Quintus-Sertorius-Guide-to-Conduct-Becoming-of-a-True-Roman-%28Redux-for-EB%29

Thanks. Will use this in my next playthrough