That's more just an AoS thing.
In WHFB there were the four Great Clans (Eshin, Skryre, Moulder, Pestilens), who were the peak of their specialisation (assassins, engineering, monsters, plagues), and then the Warlord Clans. And the four Great Clans had a lot of "thrall clans", but those thralls could be pretty loose alliances, and there were still independent clans who shared specialities. And lots of Warlord Clans who owed no one allegiance. (Mors, Rictus, Skurvy, etc.)
In fact, Clan Pestilens having one of their thralls holding a seat on the Council of Thirteen (effectively giving them two votes) was a big deal in Skaven politics.