125 should be about the absolute minimum for a ray.
I personally would aim for at least 180 gallons. Something like 6' long, 30'' wide and 20'' tall.
Any way, personally I'd keep it with a pair of indo dats. Dats usually grow pretty slow though. I don't think that's the case for teacups so care for different growth rates.
If the 125 is merely a grow out tank and you are moving the fish later on to something with at least 220-250 gallons then you could go with a black aro.