I would say a Pre-Marian army would be best represented like this; 4 Hastati, 5 Principes, 2 Velites, 3 Triarii, 4 Cav and your General. This will leave room for an artillary piece, some mercs or even another principe. Then I'd arrange them something like this;
Velites - Velites
Has - Has - Has - Has
Prin - Prin - Prin - Prin - Prin
Tri - - Tri - - Tri
CavCav Gen CavCav
Hastati were probably more numerous than Principes but more units in the second line makes it easier to outflank enemy infantry.
I generally leave the Hastati line on guard mode and also fire at will (unless they send skirmishers first.) They just hold the enemy in place.
Principes charge through the gaps in the Hastati line and flank, if possible. The two units on the extreme right and left either circle around and flank the infantry or stop enemy units from flanking you.
Cav fight ensues, the Triarii on the flanks assist. You win that, charge your Cav at the enemy archers or infantry line. Whichever is more pressing. I'd send the Triarii straight for the infantry fight unless there is more enemy Cav that they can ward off. If all goes well you should have complete control of the map and victory in the palm of your hand.
I'm not much of an online player but heres some circumstances I can think of...
Fighting a Greek army (or whatever) with phalanxes will go badly with this kind of army. Unless they are levy pikes and you have good upgrades they will tear your infantry apart. Hastati just can't handle that from the front. If they have weak pikes go ahead. If not you need to mix it up a bit and outmaneuver the phalanxes. Use pila on the flanks of the enemy rather than the front to maximise damage. Greek cavalry suck so that shouldn't be a problem. Carthage and Seluecid have great cav though. Make sure your Triarii are involved in such a fight (all of them because Pre-Marian armies suck).
Also beware of archers. They'll chew up your velites and cav in a hurry. They'll kill your infantry pretty quick too if they get flank shots off.
Skirmish-heavy armies can be hell. You need to engage your infantry quickly.
Elephants, berserkers, chariots and maybe even wardogs will screw your line up. Take archers instead of velites and shoot them. Flame the elephants and chariots see if you can make them run amok. Good infantry can also screw your line up. Keep your Gen near the infantry fight rather than away at the cav fight.
This army + formation works best against barbarians. It should do for other match ups though.
Tbh in high money matches you would be better off with Urbans than Hastati.
A lot of this was random tips you probably already know but hey!
