what happens if the floats are too short?
Part of the reason for the floats needing to be long is to give good pitch stability. If the plane is too long (or tall) compared to float length, theres a good chance that you won't have adequate elevator control to overcome even small waves, and then the plane can dig in the nose of the floats and flip over.
With a light enough plane, with the CG kept close enough to the floats (or with smooth enough water) you can get away with short floats. It just makes stability issues more difficult.