stocking options

I have a 20 and my tank is at 4 fish with coral and a cuc. you should be fine. with the flow you may want more. Im not sure what kind of 20 gallon you have either a 30" inch long or 24" long. Mine is the 30" and i have 3 power heads in there rated from 240-600 gph. My fish seem to love the flow more. I have my big 600 turn off at night to simulate the calm of night, and when all 4 of my lights come on then it turns back on. Follow the advice of putting the firefish in first, and then clowns and you should be good to go.