I wanted to get a rope fish one time for my 55 gallon and was told that I should have a group of them (I think 5 was the recommendation) or I would never see it. There is one at a Walmart near me and it seems to always be searching frantically for a hiding space.
I had some bala sharks at one time in my 55 gallon and the got too big pretty quickly. One of them (I would assume the dominant one) got to be about 10" in less than 8 months and I had to trade them in. From what I have heard, most sharks get pretty big and they need to be in groups, with the exeption of rainbow sharks, who are really territorial. I have one of them now and it spends its entire life chasing fish all day.
I have two peacock eels, which are really cool. They hide a lot, but when they come out people are in awe. I think alot of people associate eels with moray eels and dont expect to see them in freshwater. I was also told once, on this forum I believe, that a 55 gallon tank is only big enough for one peacock eel because their bioload is so large. Dont know if this is true or not, and obviously I still have mine, but they are alone in a 50 gallon tank.
In the end, I decided to stick with dwarf cichlids. They are nice in that overstocking can actually be a good thing since it keeps aggression down. They are also colorful and all that, but I find it kind of tricky finding suitable tank mates since their aggression levels are all a little different and even individuals can be more aggressive that its species is thought to be. When I bought my tank, I thought 55 gallons was huge as well and even though I was discouraged once I realized that I couldnt keep the monster fish I wanted, there are still some good options.