Yeah, histrix and reticulatas are ussually called teacup rays. But in general, any ray that is less than 6in in disk diameter is considered a tecup. Tecup really just refers to the size of the ray. But no real ray is named a teacup, it is just a common name.
i bought one retic about a year ago it was sold to me as a teacup stingray and it doesn´t grow alot mine is about 7" w/o tail only disc. when i bought him was about 4" i feed him daily with forzen shrimps.
the smallest freshwater stingray is a P. scobina it maxes out at 12-13" disk diameter you will still need a tank that is 18" wide to accomidate this ray when it is full grown. This ray seems eqasy to breed in captivity. Andreas ochs in europe has bred them
Unfortunatly you get scobinas out of Brazil. So right now there is a ban on them. The only way of finding one at this point in time, is to find a long term captive, or a captive bred ones.