If your having territorial issues, which is normal considering the fish you have, you could try a complete tank makeover 1st before re-homing. Take all the fish out, put them in seperate buckets, then re-arrange the tank completely, making several different area, each broken up by wood, plants etc, then put the least aggressive fish back in, allow them some time to get used to the tank, before adding the JD back again. It might help, probably not for long though, but worth a shot.
Or if you do decide to take back the sal and acara, dithers would include some fast moving tetras (larger tetras), giant danios possibly or some silver dollars (Be aware that SD's grow very large and need to be in groups) so maybe not the best choice for a 55g.