I read in a book about Discus care that its was not a good idea to keep them together. The reason if I remember correctly was the feeding issue as well as aggression in general. Apparently Angels are more aggressive than Discus....which I can believe. I have a pair of Angels in my tank that regularly lay eggs which they viciously guard. I can't imagine how that could be a good environment for a more timid fish like a Discus. I suppose anything can work, if you go that route I would certainly try to avoid getting a mated pair of Angels.They got along just fine in my tank. The only issue that I've read with discus/angels in the same tank is that the angels are more aggressive feeders than discus.
I agree, the Embers will end up being a pricey meal for Discus. I have Angels and the smallest fish I have currently are Bleeding Heart tetras. Pick a larger schooling fish; a school of Rummy Nose Tetras would be good for instance.I used to keep a group of 6 discus in a 75g, along with the odd angel and they were fine. Also there is no issue in terms of one passing an illness to the other, as the myth goes. They're all SA cichlids and they do quite well together ime. The 2217 is a good idea.
But the ember tetras are tiny little guys that, without a doubt, will become food for your angels and the GBRs because their maximum size is 0.75 inchs /or 2cm.