Well getting back to the main story... I suggest you dont count on local fish stores for your main info on your marine fish. They are most of the time wrong! They just want to sound like they know what they are talking about and give any old answer they can pull out of their butt. (this isn't true about every store!). A grouper will eat a damsel! thats common sense. If you didn't know that then you really have to do some internet research before buying anymore fish. I susspect you attain your info from petstores, am I right? Common mistake made too often and cause alot of expensive fish deaths. Do your own research and you'll never go wrong.
Good luck, and expect to be jumping up to that 300 gallon very soon.