I am going to buy a teacup sting ray, and I would like everyone advice i have a 75ga tank. First will it be big enough for a teacup? 2ND the tank allready has mollys (like 5, 1 inch long) and loaches (dojo, golden,& kooli) will they both the ray? 3rd question is my substrate i a small river stone, not sharp, will this work?