My perception of TexMex is that it is a style of rock, largely associated with San Antonio and south Texas, that incorporates some Hispanic influences into a rock format, and often incorporates Spanglish into the lyrics. I think that little, if any, of the music you find in those types of stores will be "TexMex". In my observations, TexMex appeals almost exclusively to an Anglo audience, with only a smattering appeal to Latinos.
The music that I have seen in los mercados is generally Tejano, salsa, Latin rap/hip-hop, popular vocal in styles such as Julio Iglesias and Shakira , and some latin rock and blues.
All of those are quite different stylings from what I perceive the OP is looking for. I can't imagine playing any of those styles of music at a venue as described unless the attendees included a substantial Mexican and Central American background.
Yes, I can differentiate between Shakira and mariachi but that's about it.
