No is the answer to both of your questions, a 20, even a 20L isn't large enough for a Leaf Fish. They don't get huge but they easily stress and with those guys Stress=Death. For two, to properly house them a 55 would be necessary and that is pushing it.
As for tank mates, if you can't eat it is stresses you, that is if your a Leaf Fish. These guys hang at the surface of weeded overgrown back or still water. They are ambush hunters and aren't comfortable in the open. Tank mates are usually eaten or cause the Leaf Fish to want to flee for more privacy. One last word, if you are not an advanced aquarium keeper then this is a very poor choice. Very cool fish but not even for the moderate level folks.