I agree with khulies and cories... both are great fish. Make sure you have at least four cories of the same species. If you want to add anything else after 3 khulies, 4 cories and 5 zebra danios, you'll have to limit the cories to one species.
Clown loaches grow to 12 inches and should be kept in groups. They are also very active fish and need to be kept with fish of a similar size. Deffinately not a choice for a 20 gallon (or most other tanks for that matter) tank.
Angels only get to about 5 inches but they need a tank that is at least 30 gallons and prefferably one 18" tall because of their finnage.
Gouramies are an option but choosing the right one can be tricky. They vary greatly in character and even individuals within the same species can be very very different in nature.
I would suggest a pearl gourami OR a three-spot (blue, opaline, gold etc.). Only one though, to limit aggression. Another option is a dwarf gourami (many varieties exist - all are colisa lalia). You could put up to 2 males in a 20 gallon without any serious aggression. You most likely won't be finding any females. However, dwarf gouramies can be rather fragile due to excessive in-breeding. By far the best choice in your circumstances would be the honey gouramies (colisa sota/chunae). Don't confuse them with the dwarf gourami variety, the 'honey dwarf gourami' (colisa lalia). These are hardier but just as peaceful and you could keep several males and females (maybe 2 males and 3 females). This large number would make your tank over-stocked, however, so watch your nitrates to determine just how many you can keep (nitrates should remain below 40 ppm or just above your tap's levels with only one weekly partial water change).
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