I do not believe a single unit could destroy the Roman legions. They would have run out of supplies (and men) before reaching the capital.
The Romans were not equivalent to the inferior Aztecs.
It would take more than a single unit to defeat the combined might (300,000 well trained warriors) of the Roman Empire.
The Aztecs also pissed off all their neighbors, so the Spanish had plenty of friends when they marched against the them...
If a relatively small amount of nerve agents were included with the Marines... or if a chemist was amongst the Marines, the Roman Empire made many raw materials from various geographies, ready available.