Normally when you have shy fish, the solution is to add some small, active schooling fish. When schooling fish are out, they make the other fish feel safer, so they're less introverted. But you already have schooling fish, and not just anything is tiger barb-compatible. Is there a hardy, small, schooling barb that could tolerate the tigers? That's where I'd start looking, if it were me. Adding bigger fish (like the afforementioned sharks) won't generally encourage the tigers to come out; if anything they'll probably encourage them to hide.
pendragon!
