I've read that Berle and Silvers were both offered and/or considered for the original Forum, which makes sense since both were gifted clowns and had Broadway experience. Of course, Silvers won the Tony for the '72 revival. I assume that Prince is just mis-remembering when he says they were considered for Fiddler since neither one was noted for being a character actor and would have been wildly inappropriate as Tevye.
I vaguely recall Buttons being a funny guy on early 50's TV, but I was only 5 and probably laughed at a lot of things back then. He also had an extensive film career, but with the exception of Sayonara his film performances are pretty unwatchable today.