they say you can keep them in a 30, which I have done with success. doesnt a 20L have the same dimensions? That said I still wouldnt do it.
Think about it this way, you have a dwarf angel in your 20 gallon tank, you have room for maybe 1 other small fish. So lets say you let your tank cycle and add 1 goby. Then you are going to add an angel to a system that has cycled and has only 1 fish? kinda dangerous....
Only way to go would be to add a couple smaller fish, then remove 1 when you add your angel. I wouldnt want to have to do that. As was posted above I would recommend other awesome fish like a royal gramma, 6 line wrasse or other things like that.
Also a 20 gallon is pretty unstable when dealing with a sensitive fish like an angel. I can see the argument for it since the swimming area is the same, but I do not think it is a good idea at all.