I'm afraid I have never owned a plec as most of the commonly available ones do grow very large and I haven't the tankspace for them . Your best option is to ask in the main tropical section or better yet, the catfish section as they will be able to tell you more about plecs than I can. From what little I do know, I don't think butterfly plecs grow particularly massive, but it's best to ask in the catfish section just to check. Post a picture of him too so they can help you make sure he really is a butterfly plec, if he isn't and turns out to be one of the bigger species, then you may have to rehome him.
White clouds are pretty , active little fish, there is a gold coloured variety as well. You could mix both colours for an interesting effect ( they will still school together as they are only a colour morph of the same fish )