I don't know about the royal pleco. I believe this fish needs more of a current than discus. According to Planet Catfish (
http://www.planetcatfish.com/ilibrary/loricari/panaque/151_f.htm ) they are fine at temperatures up to 30 degrees celsius (discus temp), but they grow large (13 1/2") and are territorial, which means one is probably a better idea than two.
Other options: I have always had great success with Ancistrus (bristlenose) in my planted discus tank. I also plan to try regal whiptails.
Zebra plecos should also do well with the high temperatures and especially the lots of meaty foods you feed your discus. I have never tried them with discus and with their price tag probably won't soon!
Cories: Most species are uncomfortable at discus temperatures. Some species are fine, but don't ask me which (corydoras elegans, I have heard, but have no experience with them). It could be a good idea to replace them with zebras.
Elephant nose: I normally prefer to keep fish from one area together, but that is only me. I have, however, never heard of anyone trying the elephant nose and discus together.
Hatchets: Lots of diskus keepers have them without any problems. Good choice. Remember they like to jump.
Cacatuoides: There are probably Apisto's that are more suited, but you should not have any problems with them. Look at some of the other Apisto's and rams as well. And they do keep to the bottom as Harry said.
Possibilities for the middle: Tetras. Cardinals (not neons), black neons, lemon tetras, rummy nose tetras.