There would be true irony if Texas were to succeed from the United States only to later hold a referendum to merge with Mexico. Succession would permanently shift the political spectrum in the rest of the U.S. toward socialism and not be a good neighbor to an independent Texas. Short of some kind of forced ethnic cleansing, the demography already within Texas today would eventually bring it back to Mexico. Texans need to carefully consider the long term consequences of this decision.
If Texas wants to merge with Mexico..., then they can stay the current course. They will not need to succeed since the Feds have ordered a stand down at the border. They are already well on their way to merging with Mexico.
However, if they were to succeed, then I would imagine one of their first orders of business would be to sweep out all the illegals.