Aquarium Masters is the only store we found in Mc Allen.
The guy there was super nice, but the store is not what you would normally expect from a lfs... especially being from LA!
They also have a Corpus branch if you are able to hit there too.
Fish Gallery is supposed to have a servicing and installation hub in Mc Allen, but I am not sure if they offer anything other than that.
Very slim pickings, and the guy at Aquarium Masters does seem to agree with you on the issue of people not being able to keep live stock in the area.
Best of luck to you and welcome!!