Carrara stores qualities (or lack thereof) for each edge in a model. Unless you create small features via small (short) edges, you won't be able to have sharp edges specified in small areas. One caveat... you can simulate sharpness with extremely small smooth edges at the expense of rendering time. Sharpness or smoothness is an artifact of the rendering process. Your model is simply a guide for the rendering engine: the model "specifies" sharpness or smoothness in the tonal averaging portion of the rendering pipeline. Boolean works well with completely enclosed spaces. Your models to be operated on should not have any holes. UV mapping will be destroyed, however. Suggest procedural shaders for any boolean objects. Good luck.