I think the Ferenggi are mentioned in one episode......they figured in an episode of Voyager, too. And, portraying a young Quezon (sp?) who wants to kill an Earthman so he can become an "adult", Aron Eisenberg recycled from his usual Ferenggi role on DS9.
One thing I think Gene Roddenberry might have missed is how much his Ferenggi resemble old antisemitic caricatures of Jewish money-lenders. I think he deliberately made them ugly because they were capitalists -- and gave them sharp teeth as visual cues -- but he may have sailed a lot closer to the wind than a lot of other people would be allowed to without being strongly challenged for ethnic slights and stereotyping. Most of the actors portraying Ferenggi seem to have been Jewish, too.