So the Treaty of Gaudalupe Hidalgo states that Mexicans living in territory that previously belonged to Mexico can remain there with full rights to their property and persons or return to Mexico without any tax penalty. It also stipulates that those wishing to remain have either 1 year to decide to remain Mexican citizens or become citizens of the U.S. If they do not decide within one year to retain their Mexican citizenship they will be considered to have chosen the path to U.S. citizenship. Article IX of the Treaty stipulates how Mexicans within these ceded territories become citizens. It also states that Congress shall decided when this happens but in the meantime all of their freedoms, rights and practice of religion shall be respected.

The right to vote wasn't established yet. However yes only white men were allowed to vote in at this point. And just like free blacks weren't allowed to vote yet so were Mexican-Americans. So even though Mexican-Americans, who were/became citizens after the Treaty of G-H, they were not allowed to vote. But in the period were are looking at poll taxes are a bit different. Post Civil-War poll taxes and literacy tests are used to prevent the newly enfranchised from voting where as in different priors years it is used as a just that a tax to generate revenue.