I wouldn't keep Angels and Discus together.
The angels are very aggresive and would bully the Discus. They are also likely to get to the food before the Discus so the discus end up being malnurished.
Somebody once told me about the parasite thing - I don;t think this is true though.`
You are also better off keeping both in larger groups to spread aggression - so big tanks are needed.
After saying all that I once saw a fantastic south american set up that had both in and they are found together in the wild................?
Mike os or Sye are the people to speak to!