If you run some of the Hastati/Principes combined with Triarii in TuGs of 5, can you also field TuGS of Hastati/Principes without Triarii in groups of 8?
You can. It is not an exclusive or (we would have used the wording "Triarii may either deployed separately or all combined with the hastati and principes" if that had been our intention).
To be honest, the whole business of how Triarii were used is very murky.
Richard