The succubus queen wouldn't have minded if all the hellhound demon king did was tag along. No one will say no to free help. Especially when the help is level 7. But demons don't do things in moderation. The hellhound demon king took over the group and became its leader by force.
The group that she formed contained four demon kings including her. All of them are level 5 demon kings. The succubus queen would have very much liked to keep it that way. She intended to put together 5 demon kings with level 5 Authority. That way, she will be the leader and she will be able to take the bulk of the wealth. She was to take 30% while the rest took 17.5%. But the presence of a level 7 changed that.