Definitely mix it up. Mysis is better than brine, but you really should add some variety in there. New Life Spectrum makes a great pellet food, but you could also do Formula I or II, or Prime Reef frozen. I'm not sure about the Gramma, (I dont have one) but the cardinals are more of a carnivore, but mine will eat just about anything I throw in the tank. Experiment a little and see what your particular fish eat
I feed a mix of New Life Spectrum, Formula II, Prime Reef, and a frozen food made by a local reefer that has pretty much everything imaginable in it
A great place for a variety of reasonably priced frozen foods is Petco if there is one near you that does saltwater (not all of them do(