There's not really enough info here to make an educated determination. Are the 500 discus all in one tank, or in one interconnected system? Biofilters produces nitrates, not reduce them. "More" biofiltration doesn't really do anything to reduce nitrates, infact they are nitrate generators. It's like the over filtration myth. Beneficial bacteria are living organisms. The colony numbers are determined by the availability of food, not the amount of surface area in the filter media, providing their is sufficient, you cannot really benefit by adding anymore than you need to support the bioload of the system.
This breeder is either not telling the whole truth about the water change schedule, or not someone in would buy from because high levels of nitrates and bacterial loads (non-beneficial) make for stunted and sickly discus. I have heard some suppliers playing down the amount of necessary water changes so as not to put people off buying discus. It has been proven here, in the general sense, that clean water is a critical factor in growth & health of discus. You only have to spend time in the fish health section to see that many of the issues people have arise from poor water quality, or not following proper QT practices.
Just my 2c worth...