How long have you had the fish? Is the tank cycled?
One dwarf gourami male OR 2 males OR 1 male with 2 females would work. If you decide on 2 males, make sure there are plenty of hiding places and some tall plants.
I'm concerned about the single cory - get a couple more. They are social fish that do much better in groups.
I'm also somewhat worried about what this 'wall sucker' may be. is it a pleco catfish? Or perhaps a chinese algae eater? Or a hillstream loach? Do a www.google.com image search for pictures of those or post a pic of what you have here. Some of the possible fish are not the best choice for a 20 gallon - or for a peaceful, tropical community.
Just sop you are aware, with 12 neons, 3 albino cories and 1 dwarf gourami, you don't actualy have room for more. I'm assuming you'll find the 'wall sucker' is something unsuitable and you need to return it and reccomending you stick to just one male dwarf gourami.