Go with black moon sand, really nice effect, though it is rather expensive compared to other stuff. A pair of gts would look nice, but so would a pair of jd's, pair of sals, trio of nics. some geos, a nice planted discus community set up, the before mentioned pike set up, a nice african theme, tons of apistogramas and tetras and stuff, and the possibilities just go on and on, lol.
Did that help? Or did it just put you into more of a dilemma? Give us an idea of what you are looking for before you ask such a general question as "I want some fish with substrate in a 55 gallon." That could be anything you know? Lol. Try to narrow down the look you want.
A nice planted community? A species tank? A pair of something? You want lots of rock work? Lots of plants? Etc, etc.