Starting a Nano

[Palster]

I recently purchased a 20 gallon Tru Vu tall acrylic hex tank for my fiancée to keep captive bread sea horses in, however after doing some research she decided seahorses might be a little too much work and difficulty for her so we decided to turn the tank into a nano. I am not sure if this tank qualifies as a true nano but I am posting here anyways. I was wondering what would be a reasonable number of fish such as gobies and clown fish to keep in a tank this size and also how much lighting would be needed to keep soft corals. I was hoping to stay away from metal halide and just go with power compact lighting at this point. The tank is 15 inches wide by 24 inches tall and is hexagon shaped.