There is actually no such fish as a teacup ray, this is just a name given by importers for small low quallity unidentified rays, usually P.hysterix. If you are considering buying a ray it is advised that you buy a larger specimin that has been in the shop for several weeks and you have witnessed it feeding.
Rays require a tank with a large footprint, a minimum of 48x24 for one with bigger being needed for 2 or more. They also require perfect water conditions at all times, nitrates must not be allowed to rise over 20ppm and the pH must be kept below 7 and stable, if your tapwater does not supply this you will need to buy a RO unit. They are known for being extremely fussy eaters and can take weeks to ween onto a good diet of frozen meaty foods, large quantities of live bloodworm can be used to keep the fish alive until they will accept other foods but will not be good for them in the long run. Like for any predatory fish large powerful external filtration is recomended, a marine style sump filter is best but 2 large canisters will do.