I rotate through a variety of foods. Vitamin enriched brine (freeze dried), frozen mysis, frozen squid, flake, cyclop-eeze (this one really gets the fish going), and live brine. for vitamins/additives, I like to use Garlic Exreme and Zoe. Soak the food in them or you can also add directly to the tank according to the directions.
As for your list of fish, you may seriously want to think about the hippo tang as they grow large over time and require larger tanks. It may be fine for awhile but eventually outgrow your 55 gal. I am debating if I can keep one happy/healthy in my 90.