A 3-foot 45g tank is not sufficient space for a Red Tail or Rainbow Shark. The Red Tail will (or should) attain five inches, the Rainbow six inches, and either require a 4-foot tank minimum. There are also other issues as these mature but I'll leave that and move on.
The Gold Gourami is the species Trichopodus trichopterus, which has several varieties/names such as the Gold, Blue, Three-Spot, Opaline, Cosby, Marble--but these are all the same fish. They attain four to five inches, but some report six inches. This is fine in your tank, but just be aware that sometimes they can get a bit nasty as they mature. You mention a green gourami...I've no idea what species this might be, can you provide the scientific name? I'm almost certain it wouldn't work in with the Gold, especially if the Gold is a male.
The Odessa Barb should be in a group of six or more, but five will likely be OK. It attains close to three inches, and is an active swimmer. Not a good match with the sedate gourami, but the Gold gourami species is one that may be able to handle this, though it is not ideal.
There really isn't much room in the upper levels with the barbs and gourami, but you could look for some suitable substrate fish (not the sharks, as already mentioned, this would be inhumane and trouble down the road). There are some lovely dwarf loach species, or the Zebra Loach (Botia strigata) that would be OK in a group of 5-7, provided you have several chunks of bogwood as loaches need this.
Now, a few comments about fish and tank sizes. Fish grow continually, both externally in size and internally with the development of the organs. The fish can become stunted in small spaces, either because of the physical space or the resulting water quality, or both. Fish do not grow to the size of their present tank as some still maintain, but they must have adequate space right from the start. Their temperament as well as their physiology depends upon this. Forcing any fish to survive in what the fish sees as too small an environment is not humane. Whomever told you that either shark could live healthily in a 20g was mistaken.
Byron.