I have 2 Clown Loaches in my 55 gallon, (well, technically one is in there, the other one is still in my QT). The CL's do have the potential to get pretty big. According to a few people on here, they take forever to grow. Their growth is slow up until they're 4" long or so, then their growth slows down even more.
I honestly think that there's nothing wrong w/ having a maximum of 2 CL's in a 55 gallon, (obviously, or else I wouldn't have decided to add them to my tank lol). But people tend to overreact when they look at the "potential" adult size of CL's and not consider that they grow at a slow pace.
As far as a suggestion for additional fish, I don't know what would be compatible w/ the fish you currently have in your tank. Dwarf Gourami are nice looking fish and add a splash of bright colors to your tank. But like I said, I don't know if it would be compatible. You could always check on that. I have a link to Petco's compatibility chart in my signature.