As marnold mentioned, we need your parameters. You should be able to find these on the web site of your municipal water authority if you are on municipal water (as opposed to private well).
A comment on your plan to "over filter." This is realistically not possible, nor desirable. "Filtration" in the sense of the biological processes that occur in an aquarium will be at the level determined by the fish load. Provided you don't over-stock, there should be no problems. Adding more "filters" in the sense of equipment is pointless and has no benefit, since the "natural filtration" will be whatever it is by itself, filter or no filter.
There is another important aspect to this, and that is the water flow. Larger or more "filters" (= the equipment) can create strong currents that many fish do not appreciate, and a fish constantly battling currents will wear itself out faster. A few fish need currents, but most forest fish (like the gourami and tetras mentioned) do not.
For a 20g long, intended for forest fish, I would use a simple sponge filter (connected to an air pump), or a small internal sponge filter that is just a sponge with a small motor head which avoids the need for an external air pump.
As for stocking, once we have the water parameters we will be able to suggest suitable fish. I would however mention that dwarf gourami is not a particularly good fish to consider. First, this species is still a danger for a disease called iridovirus, which cannot be treated; if you obtain the fish from a reliable breeder, or know the source (meaning the breeding facility, not the store), it may be safe. But dwarf gourami sold in most stores, especially the chain stores, should be avoided. The second issue is that you may end up with very small fish, and these can be eaten by gourami. A 20g long is not much space to the fish, and it will be more successful and enjoyable if you stock it with smaller fish, as you will have more of them for a more interesting display.
Byron.