I feel like the Marines would still have the advantage in any fight. Assuming that they only have infantry weapons, they still have grenade launchers, GPMGs, and disposable rocket launchers that would incapacitate several Romans at a time. A Roman testudo formation would only make the effect of fragmentation grenades and other explosives worse. If a grenade rolled into a testudo formation, that's 32-48 Roman soldiers incapacitated in the span of less than a second. Whatever training the Romans may have, they're still human, and I think an ancient plebian or equestian who hadn't seen a grenade explosion before would buck and run.
That's not counting the fact that these Marines would have advanced melee training and the ability to use bayonets, plus torso armor. If you really want to stretch, you could argue that humans were over-all shorter and less healthy 2000 years ago than today, giving a physical advantage to the modern day marines over antiquity soldiers.
This isn't to mention that it would be difficult for the Romans to organize and move an army of 60,000 soldiers, while the Marines have radios and the ability to use them, giving them the tactical flexibility to maneuver and move that the Romans couldn't match. If this fight were to take place in Europe, then the Marines would be able to use maps of the area to fight in places that nullify the Roman's advantages in numbers.
The Romans were, at one point, the most successful fighting force on the European continent. But a group of 300 physically imposing men with the ability to injure or kill dozens of your own with a metal orb the size of the baseball, punch straight through your shield and several people behind it, and out fight you in hand to hand combat, would probably make you want to run, even if you're well trained and have 60,000 men around you.
Then again, this is just an imaginary dream scenario, so /shrug.
