Are fighters allowed to use other types of martial arts in UFC, since it's called "Mixed Martial Arts"?

First off you never see Taekwondo in MMA. The reason people use Muay Thai, Boxing, BJJ and Wrestling its because they work. You rarely see these other arts such as Kung Fu, Karate, Taekwondo and Aikido because they simply don't work against some who knows who to fight in other arts such as Muay Thai. I don't care that you think you can kill a person with one strike and all this crap. Every time a Kung Fu practioner hopped into the ring with a Muay Thai guy the man gets destroyed. Kung Fu existed for over 1000 years, Muay Thai is pretty old as well and the roots of BJJ go back to the Samurai, Boxing is probably the oldest ever same with wrestling and now theres MMA to prove which ones work and that has been proven.