The (almost) Complete Guide and FAQ to Fishless Cycling - Aquarium Advice
Fish-in Cycling: Step over into the dark side - Aquarium Advice
^You have three options to start your tank out. Fishless cycling, fish in cycling, and media transfer. Fishless and fish in cycling are described in the links above. Please give them a read.
Media transfer is the easiest option as you just take a significant amout of biomedia out of somebody else's tank (a fellow fishkeeper, the store, etc) and put it in your filter. Done deal. Now you can add a small number of fish and start ammonia and nitrite free.
As far as adding salt is concerned, there are species that can tolerate some salt in the water and species that can not. Generally, salt is not needed except for brackish water fish. And that's marine salt (ie, instant ocean), not kosher/table/sea salt.
As far as keeping about 20 small fish, that's accurate, but very small fish, like chili rasboras or mosquito fish or celestial pearl danios. You only have a 15 US Gal
You can keep slightly larger (but still small) fish as well as a centerpiece.
You could keep, for example,
1 honey gourami
10 celestial pearl danios
15 cherry shrimp