So, if you buy the argument that forced assimilation is cultural genocide then, yes, there was genocide against the Mexica/Aztecs. To me, it just shows a key difference between the Spanish/Portuguese/French colonial attitude toward the conquered (partial, imperfect assimilation) versus that of the English (segregation, albeit with respect for the customs of the Indians who "weren't in the way").
Argentina is the great "outlier" in that the post-Colonial era was filled with a bitter, bloody war for control of the interior. General Rosas won that battle handily, allowing for the country to be settled by immigrants from all over Europe.
Can you recommend a book on this?