I'm probably going to sound like an old nagging granny, but you already have around 9 fish in your 20g tank.
I personally wouldn't add any more, but it is up to you. I know most of the fish are not large fish, but they still require space and clean water.
The golden wonder killifish (is that what you meant) gets around 4 to 6 inches from memory...
Anyway, to answer your question, loaches are odd and beautiful. Mainly kuhli loach and hillstream loach. But they need space and you already have too many fish.
In my opinion, the beauty of a fish tank, is to watch fish swim happily in a large tank with lots of room. They live longer and are happier and it looks more natural.
Hope I didn't sound mean :/