My suggestion, all things considering, suggest "live" sand such as aragonite, it's expensive but you can probably order it online or find it on craigslist for a decent price. I wouldn't suggest oolite sand considering your stock. I would say either 1 clown or 1 damsel and a cool looking CUC and make a nice feature tank, 20g is a bit small for more than 1 saltwater fish when you take into consideration the amount of sand and rock- minimum recommended sand bed- 2-3in and 1lb rock per gallon.
You can use regular marine sand and cultured live rock, but at this point in the game you really don't have the time to do an old fashioned cycle, lets be realistic guys
. Using live sand, cultured live rock (with as many hitchhikers as possible) stirring up the sand and adding by section and purified real ocean water along with a supplement like Kent's marine nano reef 2 part and you should be back on track, just hold onto your seat and get ready to ride out a saltwater fish in cycle
Saltwater reef tanks are the closest you can get to being God... You can watch single celled organisms collect and become life from nothing but sand, rocks and water... It's easy to understand the concept, but the devil is in the details. I don't care if you made it through grade school or not- if you can take the time and do the research, have the patience to understand the nature and design of maintaining a successful reef tank you have the equivalent of a phd in creationism, lol.