Yes, commons can grow to 18 inches. As a 55 gallon is only 13 inches wide (I believe), so you can see how that would be too small for such a large fish. A bristlenose or rubberlip pleco would stay plenty small enough for your tank, and there are other smaller species to pick from as well. I like the suggestion for lemon tetras, as well as pristella (or gold pristella), glowlight, neon, cardinal, etc. You have a lot of choices for tetras that you could keep. IMO you should google it, choose a species that you like, and if you need help then check back here for advice. One thing about BioSpira: I've never used it myself, but heard reports that it doesn't always work. Have you considered a fishless cycle? Or do you know anyone with a healthy, established tank from which you could borrow filter media or gravel to seed your biofilter?