20G is a little small for a red tail shark. He'll be very aggressive in that small of space. A rainbow shark may be a better choice, as they are not as aggressive. But you still need lots of good caves and hiding areas.
As for other fish, Bolivian Rams do well in a 20G, as well as GBR's. The Bolivians are more hardy, so if you don't want to deal with the less hardy GBR, the Bolivian is the way to go.
I would also do 6-8 tetras with them, and even a BN Pleco, as they stay small. Could do some Otos as well if it's planted, or if not, make sure you feed an algae wafer daily.
As for Bio Spira, you add that with all the fish at the same time. Will give an instant cycle if you got a good batch. Also, if you get Bio Spira, make sure it stays cold at all times til you dump it into the tank. When transporting it, it's best to transport in a cooler with ice, and have the biospira wrapped in newspaper so it doesn't freeze or kill off anything because it got too cold from sitting directly on the ice. Once home, place it in the refrigerator until you add the fish. I would add the fish first, wait a couple hours, then add the Bio Spira. That way there will be some ammonia to start with, even though there won't be enough to register on your ammonia test kit. Then just dump it into the tank, near the filter intake.
Other good fish for a 20G includes loaches, some rainbowfish, apistos, danios, cories, some barbs, rasboras, dwarf gouramis, a single angelfish (or a pair if you know you can get a mated pair), and a few others.