The tank size is very restricting here, and part of the problem. You also have non-compatible fish, as others have pointed out.
Given the tank is only a 20g tall, it is not sufficient for all of the gourami (males are territorial, and one male with five females might work, but not multiple males), nor for a proper-sized group of Serpae Tetra which have a minimum requirement of a 30-inch/75 cm long tank for a group of 15-20 Serpae, but nothing else without a much larger tank. A group of Black Skirts need a tank longer than a 20g high.
Second issue, gourami are sedate fish, and such fish should never be combined with a small fish that has a tendency to fin nip as the Serpae and Skirts both have. We all had to learn this, now you know. There are some small shoaling fish that will do well with sedate fish, but not these.
Cories do need sand, correct, if we want them to be at their best as nature intended. The reduced gourami can work with them if that is your decision. Or the reduced gourami with other peaceful fish, we could explore options if that is your decision.