Well, this could happen, I guess, but most likely it won’t. Firstly, MMA matches are separated into weight divisions
• Flyweight: under 126 lb (57 kg)
• Bantamweight: 126 to 135 lb (57 to 61 kg)
• Featherweight: 136 to 145 lb (62 to 66 kg)
• Lightweight: 146 to 155 lb (66 to 70 kg)
• Welterweight: 156 to 170 lb (71 to 77 kg)
• Middleweight: 171 to 185 lb (78 to 84 kg)
• Light Heavyweight: 186 to 205 lb (84 to 93 kg)
• Heavyweight: 206 to 265 lb (93 to 120 kg).
• Super Heavyweight: above 265 lb (120 kg).
That’s why you can’t have Brock Lesnar (current undisputed UFC Heavyweight Champion of the world) destroy UFC welterweight (and almost universally despised asshole) Josh Koscheck. Or even have a huge sumo dude against some old Tai Chi guy.
Next, while you can fight with any style you want, there are a number of styles that have been proven over the years to work really well. Here are some examples:
is something almost every single MMA fighter trains in to some extent (Except, perhaps, James Toney) it has been a dominate strategy ever since UFC 1 and has greatly increased in popularity as a martial art due to MMA
Brazilian Jiu-Jitsu (Abbreviated BJJ) is a martial art, combat sport and a self-defense system that focuses on grappling and especially ground fighting. The art was derived from the Japanese martial art of Kodokan judo in the early 20th century, which was itself developed from a number of schools (or Ryu) of Japanese jujutsu in the 19th century.
It teaches that a smaller, weaker person can successfully defend against a bigger, stronger assailant by using leverage and proper technique—most notably by applying joint-locks and chokeholds to defeat the other person. Brazilian Jiu Jitsu can be trained for sport grappling tournaments (gi and no-gi) and mixed martial arts (MMA) competition or self-defense. Sparring (commonly referred to as 'rolling') and live drilling play a major role in training, and a premium is placed on performance, especially in competition.
: The national sport of Thailand, Muay Thai is a very popular technique in MMA due to the fact it trains fighters to not only use their hands and feet, but also their elbows and knees which can create devastating strikes to their opponents.
Wrestling (Freestyle and Greco-Roman)
Muay Thai is a hard martial art from Thailand. It is similar to other Indochinese styles of kickboxing, namely pradal serey from Cambodia, tomoi from Malaysia, lethwei from Myanmar and Muay Lao from Laos. Descended from muay boran, Muay Thai is Thailand's national sport
The word muay derives from the Sanskrit mavya and Thai comes from the word Tai. Muay Thai is referred to as the "Art of Eight Limbs" or the "Science Of Eight Limbs" because it makes use of punches, kicks, elbows and knee strikes, thus using eight "points of contact", as opposed to "two points" (fists) in Western boxing and "four points" (hands and feet) used in sport-oriented martial arts. A practitioner of Muay Thai is known as a nak muay. Western practitioners are sometimes called nak muay farang meaning foreign boxer.
is another popular discipline for Mixed Martial Artists. The ability to take your opponent to the ground and while on the ground control him is incredibly powerful. While on the ground wrestling experts will either Ground and Pound (Abbreviated GnP, which simply means hitting their opponent while they are in a dominate position on the ground) their opponents, or attempt to make their opponent submit using any number of chokes and other submissions
However in the end you can fight however you want, if you want to Crain kick your way to MMA immortality, go for it. In the end, the only thing that matters is victory.