Certain soft corals (especially Discosoma and Actinodiscus) pack quite a punch (toxicity) and can easily overwhelm many (if not most) stony coral either through chemical warfare or direct physical contact. As an example, several leather coral (Sinularia spp. and Sarcophyton spp.) are believed to release chemical compounds (growth inhibitors?), which can affect surrounding coral (in particular certain Acropora spp. and Porites spp.) negatively. Quoting from Aquarium Corals by Eric Borneman under the topic Husbandry Considerations (page 112), "The obvious zonations that occur in the wild should be carefully considered, and it is in the coral keeper's interest to have soft corals and hard corals minimally intermixed, if at all." If one takes the proper precautions it can be done, but I don't recommend it.