We're getting our first chickens in a couple of days. We're starting with half a dozen. We have a portable coop that I'll move into different paddocks, a la Paul.
There's grass up to 2 feet tall in the paddocks. Does this need to be or should it be mowed? I'll be feeding the chickens bought feed and supplement as I can with grain and other food I grow, but of course that will take a while.
I think that you may want to cut the grass to height that the chickens can work with. At 2 feet tall young chickens might be overwhelmed by the grass. My understanding is that chickens eat younger grass shoots and the bugs that inhabit it. As an experiment you could try cutting the grass in one paddock at different heights and see what the chickens prefer. Let us know how it goes.
You may want to see if Joel Salatin has something to say about grass height for chickens.