The 5 bottom colonies on your planet list will be clustered together with the typical colony cluster (5 normal planets make up a C1, 5 C1s make up a C2, etc). Certain planets (U. Class, infected, assimilated, etc) cannot be cluster however.
It basically will include all of the stats of each of the 5 planets combined.. The total land of all 5, the total infrastructure of all 5, etc... although the individual planet bonuses (to population, agriculture, mining) that are unique to each of the different planet types will instead become the default of the cluster used.