A typical pre-marian army should consist of equal numbers of Hastati and Principi, with half as many Triariai, so something like

4 Hastata
4 Principi
2 Triariai

Add 2 Velites next.

Then add to that a general's unit and one other unit of cavalry only, anything will do.

Then the rest of the stack (assuming the army is a full stack army) should be made up of allies only, ie Italians for example. Historically these armies were at best 50% Roman.

Under no circumstances should an army be totally made up of Romans, unless of course one is symbolising a double legion with no supporting troops, which kind of defeats the realism aspect of the composition.