If your new percula is tank-raised, IMO
it should be eating the flakes without a problem. (my tank-raised tomato clown will eat flakes, frozen krill, live brine shrimp, live mosquito larvae, anemone poop, small nudibranchs... you get the idea) If it is wild-caught though, it probably will be pickier.
Also to add to something James wrote, you can get "enriched" (live & frozen) brine shrimp, which apparently the shrimp are fed huge doses of vitamins. This is the staple diet of my copapod-eating dragonet, as he has wiped out all the 'pods
8O in the tank.