Mouth guard for field hockey?

I just went to Sports Authority, and a salesman recommended a plastic mouth guard that was for players with braces. Now I don't have braces, but he said that it just gives you some extra protection...but when I opened & wore it it seems a little big and it's hard to close my mouth, but he said not to boil it. It fits okay but just seems to cover too much of my mouth, since the braces are supposed to add some thickness. Is that normal, or should I buy some normal ones? I doubt they'll even let me return it now though, since I've already tried it on. Thanks!