IMO, keeping a clarkii in a 20 is not a good idea. They get big (about 5"-6"). They're also very territorial and aggressive long before they reach full size. My female Clark's killed a rabbitfish bigger than she was when she was only 3" long--and she was living in a 75.
I've never kept a sixline, but I keep hearing that they're also aggressive as they get bigger, especially in small tanks.