Just get you some good live rock,at least 20 pounds but closer to 40 would be better due to the fact it will help keep the water more stable in a smaller tank like this, IMO
I would take the scallops back if you can due to their fatality rate, the sifting star if small enough could actually help if you have a dps but will be too big for the tank after it grows up. Let the water stabalize and slowly add the fish you want but with a 20 it's not going to be much.
How many watts if the light fixture on the daylight setting?
Dont get discouraged, we all go through this stage of not knowing which way to go and you will find a lot of lfs
will lead you in the wrong direction, just try and find a good one and keep on trucking.
Research each thing before adding it to the tank to avoid this problem in the future and all will be ok,