If you go check out the fish you listed at the "Fish" link to your upper left, you can read more about what their minimum tank size is, what they should take as food and so on...
Generally, for Tangs, a six foot (long) tank (usually 75+ gallons) is required, as they are VERY active and are constantly swimming back and forth in the aquarium.
The clownfish and royal gramma sound like great additions, though