A elong is serrasalmus elongata, the elogate piranha.
Focker, I see a couple problems with this,
First, why was the discus in such bad shape, I ask because it is not a good idea to feed sick fish to other fish.
Second, although I am assuming you only have the one tank for the two piranha, the elongs have a history of not accepting other species of piranha as roomies. Now that might be one way to get rid of an excess rpb but I have seen an rpb being eaten by other rpbs that managed to get in one solid bite on the way out, the fish bitten also died, be a pain to lose the elong that way, eh? Then you end up with an empty tank.
As far as using rpbs and discus as feeders, if they are excess, why not. People use red devils, convicts, and even snakeheads as feeders when that is what is surplus.