First, can you give us your source water parameters (GH and pH particularly)? Most of us don't want to be suggesting fish that may have specific requirements if it won't work due to the water.
Also, is this a basic (high) 20g, or a long 20g? That makes a difference too.
While waiting for the above, danios are too active to suit this tank and the mentioned fish (gourami and either neon, both of which are sedate fish).
The green or false neon is a wonderful little fish, not all that common due to its sensitive nature and thus specific water parameters, but if you can provide these this is a real beauty. It remains considerably smaller that the other two "neon" species and thus suits smaller tanks as it can be in a larger group which frankly is essential. I presently have 23 in my 29g Amazon blackwater tank, three of which are survivors from my former group that I acquired about seven years ago.