Predatory fish can be kept successfully with smaller species if they are fed regularly. By being fed on a regular basis, this takes away almost all reason to exert energy into a puny bite-sized snack like a damselfish for example. Also, if rocks and coral are put down at the bottom of the tank, this provides the smaller fish I mentioned above (tangs, surgeonfish, etc.) with an adequate amount of hiding space from the larger predators. These type of tropical reef fish would spend their time swimming near the coral, due to their slow swimming speed compared to their pelagic counterparts. Sure, you may lose one or two bold individuals who come out in the open every once in a while, but that wouldn't make a dent on the other hundreds of individuals who inhabit the tank. Besides, Georgia Aquarium keeps giant predators like Giant Groupers and trevallies in the same tank as smaller schooling fish like damselfish and snappers, so its proven it can be an overall success.
You are forgetting a very basic ecological rule here, predation success depends on prey population density. If you add let's say a small amount of 5.000 - 10.000 small reef fish with plenty of spaces to hide, the amount of energy required for a trevally or grouper to catch one is high. So predation will be low. On the other hand, if you really use those small fish to fill up the tank (150.000 - 300.000 seems a decent estimate, though could be even more) it becomes much easier to do catch one, so losses will start coming in quickly. And tropical fish are expensive, very expensive
As a fish-keeper myself, feeding daily also isn't the most healthy option especially for larger fish. With my small fish I already have around 1 non-food day for every 4-5 food days. So you can forget the whole "keeping them well-fed". Same case with reptiles like crocodiles b.t.w., it's done at some places but it comes at the cost of welfare.
As for filling up the tank, with the medium-sized pelagic predators and the hundreds of tropical fish, along with the Whale Sharks, manta rays, and a sea turtle or two, I doubt anyone would consider the tank mostly empty space. Also, if there is unregulated breeding in the tank, the populatiom of fish (mostly the small ones) would skyrocket to the roof.