The Marines could definitely leverage the skills of renegade or captive Romans who had technology equal to that of Renaissance Europe.
Another huge advantage they would have would be the printing press and the knowledge of how to make paper. Wars are run on ideas and they could saturate the Roman world not only with propaganda but information that would destabilize the Roman worldview.
Even though their maps might not be accurate a the local level they would geographically and topographically far ahead of anything the Romans had with accurate latitude and longitude. Their understanding of the classical Roman socio-ethnic and geopolitical situation would be abysmal.
I think their best bet would be to quickly get into the Alps into an easily defended territory and to build up from there. In a matter of a few years of preparation and, with a Hernan Cortez strategy of leveraging the oppressed non-Roman tribes and a ruthless ideology/religion to subjugate Roman culture, they would win.
It also occurs to me that the Marines could snipe the Roman generals, emperors, and anyone they wanted. The Romans would have no idea what was happening or how to defend themselves. Just a few demonstrations of modern technology and modern weaponry would seem like absolute magic to a 1st Century Roman. Fear and awe from a few artillery rounds whistling in from over the horizon might be all that’s needed to convince the Romans who lived in a world of superstition and witchcraft to submit to the new-found green gods.