I've always went for:

1x Consul
1x Legate
1x Eqvites Romani
1x Whatever allied cavalry, or 2x if far from Roma, removing the Eqvites.

1x Velites and 2x allied skirmishers.
2x allied archers.
2x Hastati and 2x allied young/poor infantry.
2x Principes and 2x allied veteran/professional infantry.
1x Triarii and 1x allied wealthy/aristocratic infantry.
1x Personal Guard unit, usually Pedites, unless I RPed something exotic.