I have used this with the adult rays but a bigger Kong would be better as it would hold more food. However the adult rays get pretty excited and maybe more than one Kong would be better. I would not bother doing it if it is their main meal time. It's just a bit of fun for them in between main meals.
Given these things are made for dogs to chew on Trev, i doubt a ray would chew it to bits. I have had this one in the tank for a few months now and there is not a single mak on it.
I was thinking of trying some kind of Hamster ball next!!