These are not “rules” but good judgment criteria. Additional criteria for selecting examples will largely depend on the topic and nature of this essay.
An engineering or paper that is pure-scientific take advantage of hypothetical cases, either well-known within the field or constructed specially by the author, to show the principle being discussed. Such examples must certanly be complete and coherent; as facile as it is possible without having to be trivial; requiring as little supporting material as possible to show the core principle, thus preventing distraction.
A practice-based topic (such as for example medicine, law, social work or business studies) should usually use real and referenced cases to illustrate a point.