Originally Posted by als1996
The tank is at 6.8-7.2 at PH and hardness I'm not sure
I would not suggest any livebearer; chances are the hardness is low, soft water, and long-term they will not do as well. The fish you now have are fine with this pH, very much so. So now to your original question.
Some fish are shoaling fish, meaning they occur in large groups in nature and must be kept in a group in an aquarium or they can be under stress which weakens their immune system and brings on various health issues they would not otherwise have. All the tetra should be in groups of 6 (or more, space permitting).
Corys do best in a group of 5, if you want more than one species 3 of each species works fine, and in a 20g you can have 6-7 corys.
Otocinclus are also shoaling, and a group of 3 minimum works best; single fish often waste away on their own; and it is important to have algae in the tank when they are added or they can starve. They will learn to eat from sinking foods in time. They only eat common green or brown algae.
Dwarf gourami have issues; in a 20g a male/female pair will work but 2 males would be aggressive. We have fish profiles here, second tab from the left in the blue bar at the top, or click on the shaded name in posts. Check out the profile of this fish to see what I'm referring to. Also check the other fish profiles for more info.
Black skirt tetra get largish, 2.5 to even 3 inches, and there isn't much room in a 20g. A smaller tetra would perhaps be worth considering, as it will allow you more "room" in terms of water conditions.