Serious pruning and cutting back

I think I have shrubs and plants like Berberis,Rhodododendron,Pieris,Hydranga,cotoneaster,Ulex, Garrya,Romneya,Rubus and many more but what I would realy like to know is how hard can plants and shrubs be cut back as a lot of mine are overgrown and I would like to get the garden back into shape.