Sorry if I'm missing anything from the discussion, I'm skimming over as I'm dizzy from a brief hospital visit.
The way I interpreted it, the rat king here represents Cazador's goal, in a way. It'd make more sense if there were 7 (or 8, counting himself) but the rat king by his own name is treated like a unit despite being made of several rats, in this case the power each soul would give him. His power would be made of many. He is one, but he is several. This is not to say he'd contain all their souls because no, they're with Mephis, but he'd contain all their power.
Obviously though there are some strong rat mentions in Astarion's story. And Cazador deep down holds himself in low self esteem, so seeing himself as a rat king seems fitting. He wants to be the best amongst the damned, yet deep down he's comparing himself to a rat. He probably fed on rats as a spawn too.
WRT the disease omen, that makes sense too, because vampires used to be very associated with diseases that took out entire families. His existence has the effects of a plague upon the city, and his plans for his future existence even moreso. I do not think he'd be content with just staying the Ascendant, he wasn't content with being a Lord either.
