The Tleilaxu are divided into several strict tiers within their culture. At the top are the Grand Masters, the genetic wizards who control the direction of the Tleilaxu, and, it is implied, hold greater stature within their religious framework. Of lesser stature are the Lesser Masters. As apathy42 pointed out, Face Dancers can assume any shape - however, Face Dancers appear to embody a glaring continuity error in Dune.
In Book II (Dune Messiah), Scytale is clearly a Face Dancer (correct me if I'm in error). He changes shape and represents the Tleilax in a conspiracy. But in Book V (Heretics of Dune), we are told that Face Dancers are (at the time of Dune Messiah) little more than drones, which can absorb the personality (to a lesser or greater extent) of anyone, and mimic their shape. Scytale is reintroduced as a Grand Master. We are also told that the Tleilax keep gholas of themselves growing on standby as a matter of course, to ensure immortality of the Grand Masters. It seems extremely odd for Herbert to introduce either two characters with the same name, and to contradict himself. The Dune Universe was painstakingly crafted and was very detailed (a fraction was revealed in the original books). For another oddity, please see Duncan Idaho.
(Of course, the first Scytale could have been manipulated by a hidden Tleilaxu, but it seems unlikely that the Guild and the Bene Gesserit would have tolerated this)