Noyesy,
I have had good luck with using Cycle. I think that there are good and bad batches of it. If you do not yet have fish, do a fishless cycle as per the sticky thread here. You should get the Aquarium Pharmaceuticals test kit with the 5 tests in it, PH, ammonia, nitrite, nitrate and high range PH. There are needed to monitor your tank, not just while it's cycling but on a regular basis. The liquid tests are far superior to the strips.
Regarding the Cycle, you can add a bit each day to speed up the cycle. All Cycle does is add bacteria to your filter which convert the ammonia to nitrite and the nitrite to nitrate. Pure ammonia can be hard to find but a dead raw prawn is much easier. Once your filter is dealing with a rotting prawn and keeping the ammonia and nitrites at 0, you can remove leftover prawn, do a water change and add fish. Keep your bottle of Cycle in the fridge.
This is much kinder to the fish, who won't have to suffer poisoning.
Be very careful to never add chlorinated water to your tank or clean your filter in it. Chlorine will kill all that bacteria and you will be back at square one.
Danios are great but glofish (as someone suggested) are genetically modified danios. Pretty disgusting IMO, to genetically engineer a flourescent fish. Have a think about whether you really want to support this sort of thing before shelling out for glofish.
But a half dozen danios, 5 cories, and a pair of pearl gouramis would be great.
Other options for centrepiece: a pair of dwarf gouramis (females are silver gray) or honey gouramis. Or a betta, but don't mix betta and gourami. Rams are nice too.
Some cories are larger than others. The smaller ones would be better in a 20g.
To get info on any fish you are considering, put the name of the fish and profile into google. Then read all the profiles. That will tell you compatibility, size, temperature requirements etc, so you can work out a good stocking. When you think you have it right, post it here for feedback.
A few live plants will make your tank look beautiful. Try anubia, windelov java fern, willow hygrophila, and wisteria. Anubia and java fern must be planted with the rhizome above the gravel ie tied to driftwood. Plant the hygro and wisteria in the gravel with a few plant tabs at their base. Cryptocorynes are also nice, they need plant tabs too.
