Personally I wouldn't recommend alot. I'm presuming you havn't got a pair because by 6" I would have expected them to have paired up (if you've had them from young).
55 gallons isn't that good of a tank due to the 13" depth, the aggression JD's have will limit tankmates. If you were to add anything I would say dithers, like danio's/tetras, though they might not last long if the JD's decide to scoff them or kill them.
If you have a common plec then they arn't really suitable for home aquaria, if possible I would try and rehome it.