Welcome to AA and Good Luck with your build!
If you keep up with maintanence, a skimmer isn't really needed with that small volume of water. Now a skimmer won't hurt by any means and if you have room and want to use 1, there are 2 choices for an "under the tank" sump skimmer or you can get a Hang on Back for the tank itself. Which you use really depends on the room you have and how your sump is setup.
I would suggest using 2 heaters(redundancy). You can put both in the sump.
Dry aragonite sand is a good choice. I use the real fine oolitic sand from CaribSea. I'm not a fan of deep sand beds, so I just have a layer about 1"-2". If you do use a DSB
, make it at least 6" deep.
If you don't plan on any corals, a stock 24" flourescent bulb from HD or Lowes will work fine. I use a 6500k bulb from HD for over my 20g long qt. If you even think you may want to try corals, look into T5HO fixtures.
When you say trickle filter, do you mean a wet/dry with bioballs? They definitely do work. I used that setup for over 15 yrs. The main thing using bioballs is to make sure the water coming from the tank into that section is free of debris. A sponge in the overflow will work, but that sponge must be cleaned often.
The recommended flow rate thru a sump is 5-10x the tank volume. A 20g long doesn't really hold 20g and with displacement from the rock, depending on how high you run the sump, your total volume may only be 25g. So then you want a pump thats rated anywhere from 125 - 250gph after you figure the head loss from the distance the pump has to push the water up into the tank.
As far as your fish list, cross out any pygmy angel. Your tank is too small.