Im gonna agree with DB here.
We've also seen smaller variants of black rays at a very large size as compared to what we would see of their larger cousins. I think that both will get to the same size.
Any type of ray is good for breeding, but if starting money is an issue, breed regular motoro's, theres nothing wrong with that. Unless you are harboring a very large facility, it is going to be difficult to harvest a profit off of breeding them. And its no quick process either.
retics are kinda pointless..... but
So my summary is, get three (if you can properly house them and do your research) of the kind you want to own, not just the kind that may potentially make you the most money.