i think its a great idea. a lagoon ecosystem tank would look great. as said above mullet in general are grazers feeding off algae and micro-organisms in the top layers of sediment on the ocean floor. but are also opertunistic in their feeding habbits. ive seen mullet caught on bread, what are described as bristleworms in aquariums (ragworm in the uk). shore crabs, various mollusc's. ive even seen a group of over a dozen mullet feeding on the carcas of salmon, some one had filleted at the jetty, and had discarded. so the mullet we have in the uk (Chelon labrosus) will eat pretty much anything. as far as im aware, they will have the same feeding habits as the species you intend to keep.
i think you'll need a big tank, not stupid big, but wide enough so they have pleanty of swimming room. i also think a pretty substantial refugium is a must, to try to provide some of the micro-organisms on which their diet is based in the wild.
good luck. keep us posted