I have a Plec, and three Clown loaches. The plec is great for cleaning the bogwood and glass, but the loaches clean up the gravel. As far as being nocturnal, I've found that the plec will hide all day every day, and the loaches did the same for the first 2 months I had them, but in the last 3 weeks or so, they've started to come out and play during the day, I even had one taking bloodworm from my hand as I put it in the tank today!!!!!
A cool sort-of bottom feeder I have is a blind cave tetra. If you're into fish that look like they're about 3 days dead and had their eyes eaten out, you'll love them.
They hunt by scent and seem to eat most anything they can find on the bottom (they will eat from the surface too, but mine doesn't have a chance of getting anything before the sharp-eyed tiger barbs). They don't get that big and they're hard to kill. Probably because they're zombies.