Temperment: Semi aggressive, keep them well fed and you should be ok, if they are hungry they might take out any tankmates that look tasty.
Compatability: Peacefull larger fishes generally make ok tanmates, with the more delicate rays a species tank is prefferable.
Diet: Any large meaty foods, mine eat shrimp, cockles, clams, mussels, fish (not live) earth worms and large snails
and size: The smallest species will grow to at least 12 inches across, unfortunately these are not commonly traded and the ones you are most likely to find will grow to between 15 and 24 inches across.
Oh yes, also wondering if they are venomous? Yes they are venemous, dont get stung as it apperently hurts like hell and will need hospital treatment.
Freshwater rays are fairly delicate fish that need a bit more care than most fish, large and regular water changes are a must and the substrate needs to be kept clean at all times, remove any uneaten food from the tank within half an hour of feeding them and make sure the filters are running at full capacity always.