Probably.

Then it becomes a question of hegemony. Would Romans+allies have enough power to win?

And I think my earlier conclusion is the same.

The scenario seems a bit unrealistic to me.
but would the romans have gunpowder? then their armour and tactics would change.

I'd say in a field battle with even conditions and numbers Romans would win because of superior technology.

It seems silly to ask "would the Romans have gunpowder?" If they didn't have it when the Roman empire was a going concern, I don't see why they'd have it in this hypothetical situation.

Gunpowder was invented in China in the 9th century. It didn't reach Europe till the 12th century. The Roman Empire 'fell' in 476.

Hypothetically, at the most I'll give you (maybe) early, early firearms to work with in this discussion but only in limited usage on the Romans part.

But those weapons were slow and cumbersome, and if the Romans fired, hit a few and reloaded with a huge mass of Aztecs charging with weapons they didn't need to reload, the Romans would have been screwed, so firearms would really be counter Roman success.

this is a ripoff of my thread!!!
Romans vs China ring a bell?

It ain't a ripoff if its only half-related.

Read the first post here

The advantages of early firearms versus civilizations without firearms was the entire scare-factor. Early French military campaings in New France come to mind. In one encounter, Samuel de Champlain, 2 other Frenchmen and a small Huron war party encountered 200 Iroquois warriors, Champlain shot 2 of the Iroquois warchiefs with his arquebus and the entire Iroquois force fled.

In any case, the Romans wouldn't have gunpowder as the Roman, and later Western Roman Empire pre-dated gunpowder weapons in Europe. The Eastern Empire would have encountered some form of gunpowder weapon prior to its fall in 1453, but at that time (and pretty much from the 7th Century on), the Eastern Empire was then the Byzantine Empire and could hardly be called Roman.

I still hold that a properly-supplied, armed and more importantly, commanded Roman army complete with cavalry and possibly ballistae would have annihilated a Mesoamerican army of the same era, should it be fought on a fair and evenly balanced field of battle. Should it take place in deep jungle, the Romans would have initial setbacks, but the Romans as always would have adapted and overcome, much like they did on the high seas during the 1st Punic War, or on land during the 2nd Punic War.