the only trouble with plaster of paris is that it will crack if not applied thickly enough, it will go off (hard) pretty quick too, so you'll have very little time to mould anything if thats what you want to do,
Plaster of paris on paper mache works well but you MUST make sure all the paper is totally dry before sealing it of or it will rot underneath and eventually collapse. Over chicken wire (no packing under the wire) it would be hard to get the plaster to hang on to the wire and even if you did you'd have to do multiple layers to get some decent strength to a hollow object.
Its great where a smooth 'non rock' look is wanted, i've used it over the top of render to get a really smooth look and the coats only need to be thin so they dry really quick.
@bigwilliestyles, unless its sealed it wont be any good wet (thats why they use garbage bags over broken bones while showering)
Polystyrene on its own isn't strong thats why most of us use several coats of render to strengthen it, using the foam is just a kind of form work, to support the render.