I always thought of it as more of a separation of types for different jobs, such as with bees. The 'drones' would be the soldiers and messangers while the 'thinkers', such as Shivkala on Centauri Prime actually ran things. Technically, the Keepers were Drakh, too, and an outsider would think that they were a separate race if they didn't know that they 'budded' from another Drakh. Wasn't it established in the novels that the Drakh have something of a hive mind?
You know, there's definitely more about the Drakh in the Legions of Fire books, but I can't remember anything off the top of my head.