I wouldn't mix angels and discus. Discus prefer warmer temps and are less aggressive than angels. Discus require clean pristine water and do need regular water changes. Larger tanks are easier to keep stable than smaller tanks. Young discus should be in groups of at least 5 and the smallest tank I personally would have discus in is 55gals. Discus are a beautiful and rewarding fish but do require patience and a bit of work.