So this is probably pretty obvious to everybody, but given the delay i've just rediscovered the idea;
Kyuubi is gonna try to fuck us over once we get going. of course. Three things in life are inevitable, death taxes and that little shitstain screwing people over.
But the method he'll use is to support us all the way. No really, he loves us, totally helpful always! And we survive to eventually conquer the city, fill our team up with 20-30 girls, a mix of converts and local newbies.
But Kyuubi keeps contracting new girls. Not at a malicious rate, but steady enough that we cant keep up, the city doesn't have the capacity. We object, but he explains that this girl was in a car wreck, that one was on the edge of suicide, he's saving their life by granting their wish, we wouldn't want him to let them die would we?
So we're full, we're forced to expand, and Kyuubi 'helpfully' supports us as we push outwards. Naturally, this means everybody else is suspicious of us and hates us. The more Kyuubi likes us, the worse our PR gets, but we have no choice but to conquer as our girls starve.
So yeah. The empire of all of japan isn't a choice, its an inevitability.