mag-10 while not gaining weight

Im a athlete, who once in a while compete, so I cant use illegal stuff. I guess this has been up before,but I cant find the info I seek. Since mag10 is not a classified supplement,do I run the risk of testing positive, and can I stay the same weight, but still gain strength?
If I do gain too much weight, will there be any good chance for me to loose the weight, while retain most of the strength (otherwise it wont be worth it, now will it).