Actually, Mewtwo had indeed been planned for 64, along with Bowser and Dedede. However, Meowth, Pit, and Peach have never been stated by Sakurai to have been planned. See the first fact here, which is properly cited:Wrong.
Meowth and mewtwo have never been confirmed by Nintendo to be in any kind of development in 64. iirc, bowser and king dedede were the only canceled characters that were actually going to be in the game, but there has been speculation over things ranging from Pit to Meowth, all of which are undoubtedly loose speculation.
http://forum.starmen.net/forum/Stor...ing-a-comprehensive-list-of-unused-characters