[Tonz of fun]

Sounds like you sould get a bigger tank bud. I have learned Two things. One do it right the first time, two take your time. First question I would say at least 30 lbs up to 20 lbs of rock. In terms of placement that is the beauty of setting up your tank. You get to be the artist. I recomend putting it on the bottom of the tank not on sand becaus it will move on you. I personally feel that a tang will not like that small of a tank others will not agree. A rule of thumb is 1 4 inch fish per 10 gallons. This means you could have a couple small fish. Tangs are delicate and big plenty of swimming space with caves and lots of algea which means it should be a developed tank. What kind of lights, filtration, and skimmer are you going to have for the corals?