How about this division of naming for consistency.
Primary Civs: The core Civs that are in vanilla RFC and the game really won't work without, (ex. China, "vanilla" India, Egypt, Persia, Rome, Arabia, Mongols, etc.)
Secondary Civs: The new DoC civs and respawns, what Leoreth referred to as not critical to the core game (ex. Poland, Tamils, Moors, Italy, Thais, etc.)
Those two categories can be referred to as Major Civs.
Minor Civs: Seljuks, Celts, Natives, Indies off the top of my head, those who are scripted