bertoni, correct me if I'm wrong, but I think what you're getting at is there's already a sufficient supply of nitrifying and denitrifying bacteria living on and in the rock, so there's no need to introduce ammonia to the system to get the population going.
As for cycling, even when it's not necessary, I think it's necessary. Here's what I mean by that. Even if everything should theoretically be in balance when you start the tank up (using pre-cured/cultured rock, sand, etc.), would you rather deal with any potential problems a new setup may pose with or without having to worry about a bunch of livestock. Your problem may not even be biology or chemistry related. What if you have a small, undetected leak in your plumbing that doesn't become a big leak until the tank's been up and running a couple weeks.