Italians in the legions

[Frank]

As well, if the legions are manned by non-Romans/Italians, ...

The legions were never manned by non-romans (except some special legions during the civil war). Non-romans entered the auxilia. Non-Italian does not mean Non-Roman. The roman population outside of Italy grew, while the italian population stagnated or even decreased over time.

Nathan mentioned already one reason, others were:
- Citizens of the provinces getting roman civil rights after 25 years of service in the auxilia
- Local elite of cities and tribes in the provinces(Curiales, Decuriones) getting civil rights usually pretty early and so all their desendants have been romans
- sometimes entire local cities getting civil rights
- a lot of roman colonies were founded outside of Italy, which also decreased the italian population.

And even inside Italy the main recruiting area was northern Italy since the late republic. A lot of these romans had celtic roots. The romans have been always convinced, that people from rural areas are the better soldiers. They are used to discipline and a hard life. And looking to the structure of italian agriculture, it is obvious, that you find more recruitable rural romans outside of Italy.