The smaller discus would be better left in the smaller tank with up to 100% water changes daily until they grow. They will be more healthy, grow faster and get more food this way. I have 5 bigger 10cm plus discus in a 450 litre planted tank with neons, cardinals, rummynoses, corys, bn and hatchets and they still manage to pick on the smaller one of the 5 and fight daily.