Depends what water conditions he plans to keep his discus in. If using RO and planning to reduce the pH to something closer to Discus wild conditions (6-7) It wont be ideal for alot of synos. If you want them in soft water conditions choose carefully and acclimatise very slowly. I still think the best choice would be corys.
As for plecs, well most will go fine with Discus, I would choose a panaque species as then you dont have the risk of the plec sucking onto the side of the discus to eat the slime.