That is a very nice aquascape. One thing lacking are floating plants. This has quite an impact on the fish. Small "nano" fish are suited to this tank, but they will be skittish and nervous without cover. I don't see a problem with kuhli loaches, but you may never see them. Many aquarists find they remain out of sigh during the day (tanklight on, with open water like this) and only scamper about when the light is off. This could hold for many fish. Floaters would solve this.
I know many think they need "centrepiece" fish like gourami, betta, etc, but this does not work very well in most situations. Groups of small fish provide more active interest. A 20g is still pretty small space when it comes to the fish.