Nice worm farm! I have too many other projects going on so I'll hold off on the worm thing myself.

As for general composting and rats, we follow Salatin's example. He puts a large amount of carbon on the bottom and sides of the compost heap so rats won't smell whatever is in there.
Animal brains work by the concept of effort vs. reward meaning that if it's too easy to get to they will do anything to get there. Like raiding a chicken coop.
But if it is too much effort, like digging several feet down, they won't bother. Same goes with your average mobile animal setup, the predator has to go to a new place (since you move them) which is a danger on it's own but also has to expose himself to threats like birds of prey (if you keep the surrounding pasture short).

Developing a sense of the effort vs. reward concept can really help prevent trouble with these critters.