Ha ha ha, I think you should resort to your principles more often.

When I play Rome, I rarely even use the manipular formation.
In fact, in all games I try and fight with a different style, as though that faction had suddenly undergone a military reform.

But to advise you, you aren't meant to wait until the Hastati rout before engaging the enemy.
They are meant to slowly give way by degrees, and then retreat behind the formation.
Same with the Principes.

Knowing Roman units in EB, it is doubtful a single celt will engage with a Triarius.

Oh, and it's ok to have 3-4 units of Auxiliaries - these will be your best, and more historical flankers.