There is no such fish as a tea cup stingray, this is just a generic name used by importers and lfs for small unidentified and often low quallity stingrays, usually P.motoro or P.hysteryx.
Teacups are not a species of ray and the name is used to describe rays as CFC said and also as a common name for a particular size, of small ray (these could be Retics, Humerosas, or Histrix...). Why this is called 'Teacup' I have no idea as the rays are normally 8"+ not 3"!
Yes, he is. He's really small right now so, he's okay. And I don't know what species he is right now because he's reffered to as a "teacup". And 8'' is his MAX size.(not that big) so, he's okay in the 30. Whatever.
I must stongly disagree, the smallest species I know reaches 12" in diameter of 30 cm. These are Potamotrygon histrix and Potamotrygon reticulata, are these your rays? A 30 gallon is no were near big enough for a ray, I would recommend at least 75-90 gallons with a big foot print.
I'm really not sure what kinds of rays you're talking about, but the ray I bought from the store has an adult size of 8 inches. But, if it gets too big, like you seem to think, I'll buy a larger tank.
Well Potamotrygon sp. sometimes called a Chocolater Ray has a disc up to 18" wide so you would need an even bigger tank than I first suggested. Is it possible for you to get a photo?