First of all nice work on the rock! It looks great. I think with something as small as a 20g you will be fine with with the skilter, especially with your sand bed and rock, which will eventually aid in the filtration. You won't see much waste in your skimmer for a while because there really aren't any dissolved organics as of yet. That will be a sign that your tank is starting to cycle. Get a marine basic test kit (ammonia, nitrate, nitrite, pH) and test twice daily (right after lights on and before lights off). You will see certain trends that will tell you how your cycle is going, which is VERY important.
Like mentioned earlier, mushrooms would look well in that tank, will work well with the above-mentioned fish, and are very hardy and ideal for beginners. Of course don't add for a while AFTER your cycle completes.
You've been through the ringer about the damsel, just my .02, let it die. Damsels are the biggest pain in the A$$.