imo you can top it, top it, and top it some more, but i would not prune anything. i only remove leaves that are 50% or more damaged or that are seriously covering other big colas. the plant already put the energy into growing them so let the leaves do there job and create more energy. the bonzai look you are talking about is achieved by topping the plant a bunch or by growing from clone. i can tell your plant is obviously from seed by the way it is growing. how much room do you have for vertical growth?