Hivemind is gooood.
It was basically the same mechanic used for Cranium Rats in Planescape 2e, and those creatures were the invisible puppets behind *many* schemes. They were, after all, the informants and information gatherers of the Mind Flayer God.
There was even an NPC hivemind, called The Us. Their ultimate dream is to "liberate" every rat by adding them to their collective. I could see Moonrats doing likewise.
And why would they kill, when the can dominate?
To become more intelligent, when they could, I could imagine them stealing magic items from the local shop -- many headbands of Intelligence, rings of wishes -- the more potent and permenant the gain the better. One point of Int gives them enough to start taking class levels, even in their off time. Hungrily gathering more intelligence, scrimping and saving, getting enough to charm or even eventually dominate the odd ratcathcer, to persuade them to add more to the ranks. Perhaps even cats, owls, etc. can be dominated in that way -- pehraps some of the rats take levels in Druid, trying to get to the point where they can awaken their brethren.
To facilitate this, maybe they set themselves up as Wizard's familiars for a while. On the odd chance the wizard gets some intelligence-boosting item, *yoink,* and they're gone, along with the pet.
Always add more. Thirst and hunger for genious. Train wizards, train druids. Awaken when they can. Never succumb to the darkness that waits for the fading moon.
Also, I must say that the idea of your fairy-tale campaign sounds pretty sweet.
