Comparing Mexico and China in assimilation of their conquers isn't really accurate. The Chinese had overwhelming numbers while the Mexicans or the Columbians have smaller population than the US .As one the previous posters have said if taken earlier enough like before the population boom of Latin America the US could assimilate those regions.
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Let's say it's true, with I don't have the numbers to check. Then what? Do you think that many anglo settlers would want to move to Central American jungles or Colombian highlands?
On the other hand, we must bear in mind that, even if the population was lower than later, these peoples were farmers, their population density was way higher than that of Native Americans in the Grat plains, and they were familiar with modern warfare and guerilla tactics: they would vehemently resist any attempt of settlers to take away thir land, no matter if settlers come with a paper saying they bought the land from absentee Criollo landowners.
This population would survive, in their land and with their religion and culture. And when the population boom happens, they would move to big cities in firmer Latin America and then in the original US territory, carrying with them their languge and culture.
Hispanic population boom would happen no matter what. Remember IOTL after the local population was affected severely by diseas and conquest, at some pint in the early XVII population established and began growing again. If they are conquered by Americans around 1800, eventualy modern medicine techinques would arrive, and you'd see a population boom even if the regions remains poor, as it has happened in Africa or Southeast asia IOTL.