Bio filtration is the removal of toxins such as ammonia, via bacteria (ammonia -> nitrite -> nitrate) and via plants, which remove nitrogen in all 3 forms. It is also the removal of other things that we generally do not test for or list. Plants are good at this. In a planted tank there may be less nitrifying bacteria compared to a non-planted tank because the plants are removing a certain amount of the nitrogen.
The microrganisms of the nitrogen cycle will only grow to the limits of their food (ammonia).
If you add more of these microorganisms (lets say you dump in a large bottle of Dr. Tim's One and Only) than there is food, they will die off until they balance the food supply. In dying, they feed the ones that are still alive, so the population goes down slowly. It won't usually die off so fast as to cause an ammonia spike.
So you cannot have too many of these microorganisms- they will die of starvation.
You can add more and more plants into the system. Eventually the plants will be removing all the nitrogen and other things that they can (usually shown as a NO3 test of zero), and will be deficient in nutrients, and stop growing. You can add fertilizers to supplement them.
The filter media that the micoorganisms prefer have lots of surface area, and the right pore size for good water movement. There are many species of microorganisms, not just the nitrifying species that live together in a complex web in a biofilm. Optimum water movement will bring oxygen and ammonia (and other foods) to the many species of microorganisms in the biofilm. These organisms will live on all the surfaces in the system that suit them. Optimum water flow, low light and so on.
If you have too much filter media that is labeled 'bio media' then the organisms will only colonize the spots that are the very best, and only a few organisms will live on the areas that are less than optimum. Bio media is not generally very good at debris removal.
If you have a lot of mixed filter media- sponges, floss and other things usually thought of as mechanical media, then the bacteria will colonize the areas they like whether you call it mechanical or bio. No matter how many microorganisms grow on these media, you are getting the benefit of both mechanical and bio filtration.
Net comment:
If you have a lot of mechanical media, you are getting the benefit of the debris removal and the microorganisms.
If you have a lot of bio media, you may not have enough mechanical media. It may be only partially colonized, too, since the plants are doing a lot of the bio filtration.
So, lets reword the question:
Can you have too much bio media?
Yes, especially in a planted tank where the plants are doing a lot of the bio filtration. The excess volume of bio media may take up the space in the filter that could be better used by more mechanical media.
Can you have too much bio filtration organisms?
Microorganisms will die off to match the food supply, so no, you cannot have excess of these.
Plants can grow only to the limits of their food, then may die, stop growing or look bad. They can crowd each other, shade each other and so on. If you are willing to take care of the plants, trimming, adding ferts as needed, and grow the ones that fit in the tank, then you cannot have too many plants.