Discus will not go with large SA, CA or African cichlids well at all. The discus will end up falling ill, getting beaten on, and then die.
By the way, a 55gal is fine for 7 adult discus, you just need to make sure your doing a rigorous water changing regime, 50% every day or 75% every other day.
Being so new to the hobby, or at least seeming that way, I would not reccomend you buy fish like discus who are relatively touchy, and pretty expensive.