Real cinder is not advisable. While it won't lower the pH, it may leach toxins into the tank. You can try it out in a tub of SW--test the water, soak the rock for 3-4 days, test the water again.
You really can't have too much bio-media. What you can have is too many places for solid debris to become trapped and inaccessible to larger scale cleaners, which will then increase the ammonia produced (and the nitrites, and the nitrates). But, this is because there's too much food for the bacteria, rather than because there is too much bacteria.