Instead of thinking of a centerpiece fish right now I would consider increasing the schools of your neons, lamp eye tetras, and glo-lite tetras. They are all schooling fish and will do better and be happier if they have more of thier own kind. I would consider increasing the school of each one of these fish to a minimum of six with more of each the better option. According to your sig. you also only have one cherry barb. Although the cherry barbs are not considered schooling fish, they do like more of the same to hang with and play togather. I see you have an angelfish listed. I would think that the angel makes a great centerpiece fish.
I might be wrong, but don't Dwarf Sting Rays, Clown Knife Fish, and scats get to big for a 55 gallon and I don't think they would be compatible with the fish Freshwater_J has.
scats would work except they bw and dwarf stingrays might be illegal in his state. most cool oddball fish would eat your neons, but you could get an african butterfly fish, which is a mini arowana that get about 4". o yea and your angel will eventually eat your tetras.