do the festae or you could do some HRP's and i don't care what most people here say most of the fish stated above can survive perfectly fine in a 55 especially because they are alone and nothing else that is large in the tank...so don't listen to them if its solo its fine aslong as it doesn't get longer than 12 inches or so.
if you want the prettiest freshwater fish there is (imo) then id say a festae. the females maximum is 12 inches but they grow real slow and most stop around 8 inch. male to big
It is untrue that a Goldsaum (green terror) will be much happier in a 75 gallon. A 55 will be just fine for one of these for life. A Goldsaum is my vote.