According to the AAMC you are not a URM. You have to be Mexican or mainland puerto rican. You can only count as Mexican American if you are "MEXICAN"

. You can apply as economically disadvantaged if you are poor though. If you apply as mexican then they find out you are something else, they might see you as being dishonest. It is lame how they catorgorize some hispanics as URMs and others as non URMs. I don't know if medical schools really distinguish between whether you are cuban, mexican, etc. If you feel that you are disadvantaged in any way (non native speaker, being economically, and/or educationally disadvantaged) you should definetly discuss it in your personal statement.
I heard that the AAMC might have changed their policy distinguishing who gets URM status as those that are underrepresented in medicine (whatever that means). I would call the schools that you are looking at and ask them. Good Luck!