The one exception to absolutely never using "y'all" in the singular form is if you are talking to someone who is a representative of a larger entity. This is absolutely the only acceptable time to use "y'all" when talking to only one person! Otherwise, you use the word "yer" or "you". Incorrect use (or absence) of the word "y'all" in Texas could mean you have seriously insulted someone. Usually in commercial transactions. For instance...
To ask the lady at the cash register of Western Warehouse if the store will be receiving any new Tony Lamas (the only cowboy boots) soon you would say: "'scuse me miss, but when're y'all gettin' some Tony Lamas in?". Because the subject of this sentence is not the cashier, it is that particular store, and all of its included staff. If you were to ask "...when're you getting some Tony Lamas in" she would probably take it as an insult, because you are implying that she herself will be receiving Tony Lamas inside herself... not a very pleasant thought for most people.
But if you wanted to compliment her mullet you would say: "Yer hair sure does look purty today." If you said to her "Y'all's hair sure do look purty today" that would be taken as an insult because the staff of Western Warehouse most likely has some men on its payroll, and if you are in Texas you never tell a man he's pretty unless you're mocking him. And if you mock him, you'll probably find out real quick just when and where you'll get yer Tony Lamas.
Of course, if you're in Texas, there's a good chance that you'll be given a funny look when you say these things, and then answered in Spanish (not a joke, y'all, you don't believe me, then get yer arse down to Texas and try it). So be aware of who your target is. If they speak English, and are "Speaking Texan", then you'd better know proper grammatical usage of "y'all"... or you might end up dying from lead poisoning.