Let me say this about TKD and the problems with bringing it into MMA.
b/c his Friend Knows TKD.
TKD is good in mma if you can use it, but it's hard.. i use my TKD a lot in my fights and i work hard to adapt it
I've fought a half dozen TKD blackbelts in MMA/pankration/NHB competition, and there are alot of reasons why TKD is a waste of time in training for MMA.
The first is that they focus so much on point sparring that as hard as they hit me, they can never do any serious damage. A great boxer (like Olympic Gold Medalist Andre Ward, who I've worked with a few times) hits quick and hard, but TKD guys lack the input from their hips that allows them to do alot of the serious damage that a boxer can do with their hands and that a kickboxer/muay thai fighter can do with their feet and shins and knees.
It marginally increases their speed, which matters alot for point-sparring competition, but it doesn't matter at all in MMA. Landing one good shot that puts an opponent to sleep is much, much better than landing a couple dozen that do next to nothing (except, they tend to make your opponent angry and give them a great chance to close distance and take the fight to the ground).
The obvious one (as we've seen from guys like Danny Abbadi) is that they have no ground game. Even with guys in as extreme a martial art as Kajukembo (and Grandmast Gaylord is a genuis, despite his funny name), if you don't teach submission skills, you're going to get tapped out.
The only TKD guy who's ever given me a problem is a guy from Pitt's Martial Arts acadamy, and his instructor holds blackbelts in 5 martial arts, one of which is JJJ (which he teaches out of his school). The rest all struggled to hurt me and they all failed to put up much of a fight on the ground.
If you're going to learn a martial art, I also suggest going into something that has a background more focussed on self defense, or at least full contact competition. I tend to frown on any style that uses point sparring as a means of judging the capacity of competitors, as it's not really an accurate meter for judging anything.
I also feel that blackbelts in TKD (and many karate styles) are given out way to easily, but that's just coming from my background as a GJJ guy, where getting a blackbelt is a mark of serious mastery and getting a purple belt is considered a serious achievement.