No, going for that path I only get Rat Cyril joining me, and nothing else. The Rat King stays. Spit-roating Cyril only leaves him at the rat's den, not unlike what happens with the Guardian, though the Rat King joins me in the latter scenario.
Then at the moment it isn't possible, sorry. In later updates the rat clan will hold a lot more quests and mechanics around hierarchy, so rest assured you'll have other paths to get the Rodolphe (the rat King) to join your party.
