Don't put anything too small in there. I have six Discus with 2 Silver dollars and I put 12 neons in with them. 2 Days later I only had six left and so I moved them into my small tank.
I told the guy where I bought my Discus and he just grinned and said it will have been the Discus.
I'm not %100 though it could have been my Silver Dollars, they're pretty big.
Neons can't withstand the higher temps of Discus life and to a fully grown Discus they are food. IMO the best are the Rummy Nosed Tetras or Cardinals, both wonderfully colourful and make a great shoal in a planted tank. However, Cardinals don't tend to school aswell IME.
I dont have Discus, but my LFS has a 600 gallon display tank with Discus, and they have both rummynose and Congo tetras in there with them... I do have Congos, and they are IMO one of the prettiest tropical fish there are.