Politically correct answer: Sure, keep practicing what you like, adding the GC will just make it work better. . . .
Problem with that answer: It will never allow you to discover your own full potential.
My personal story:
After doing the kiddie Karate and then Tae Kwon Do black belt thing, I realized after some close calls in my mid teens that I hadn't really learned anything practical. I started researching and found Jeet Kune Do (JKD). I picked up a bunch of videos and went nuts with friends, beating on each other wearing motorcycle helmets. Prophetically, one of my training partners took off his helmet after a clash and said, "Jeez, was that as CHAOTIC as it felt???"
Couldn't find a JKD school in New York City at that time, but managed to make a couple seminars in Connecticut and New Jersey. Good times. Then I walked into a Wing Tsun class, thinking I'd stay a few months and pick up some Chi Sao (sticking hands) skillz. . . . Got completely controlled the first night, completely knocked out the second. Seven years later, I was the second most senior instructor in NYC, and I also taught Escrima (Filipino martial arts that trains primarily with sticks and knives) with the same organization. Neither the Wing Tsun nor the Escrima were anything like what I'd been led to expect while I was involved in JKD.
Although the Wing Tsun training saved my ass several times, I began to feel uncomfortable about it. Reality never went the way I expected it to go, the way it had gone in training--even though I was successful. I started to look around. Got involved in Russian Systema, both regular classes and a few seminars with master instructors. See Attack Proof Newsletter #90 for the story there. A chance meeting with some long time students of Charlie Nelson (WWII Marine and self-defense instructor), during which I was soundly smashed for asking stupid questions, got me involved in the close combat community, in particular Carl Cestari's crew. I also began practicing Brazilian Jiu-Jitsu and Mixed Martial Arts, after a few months of Judo.
Things were rather nebulous. Some Wing Tsun students were complaining about the close combat attitude I was bringing into the classes (although some loved it). The Wing Tsun and Escrima classes I had been teaching at Columbia University had morphed into close combat/self-defense classes. The BJJ/MMA was keeping me in great shape, although I felt it was a bit limited in terms of real-life practicality. Carl Cestari's crew had only occasional sessions in Jersey, and I wanted to get some quality hands-on close combat training on a regular basis closer to home. . . . That's what got me into a GC class for the first time--the close combat aspect. I discovered there was a lot more to it though. Much of that journey is documented in the first few years of the Attack Proof blog.
Regarding GC and other martial arts:
Besides Systema and GC, all martial arts that I know of attempt to train certain movement patterns, structures and habits into the student, claiming that such patterns, structures and habits are the "best" for dealing with violence. This includes JKD, except that the exact patterns, structures and habits each particular JKD teacher teaches vary widely (Original JKD, JKD Concepts, instructors' and students' preferences, etc.). They're still, however, teaching movements and then attempting to apply them.
GC is different in that it acknowledges that violence is so chaotic and human movement so variable that there cannot be any "best" moves or patterns independent of the complete context of each unique moment of each unique situation. To attempt to deal with the fluid chaos of real violence with a reductionist set of trained movements and habits will result in one's never really perceiving nor being able to move effectively with the reality of a given situation. All one can do in such a case is hope that one's superior attributes will allow him to force his round and square pegs (his trained movements and habits) into the jagged, irregular holes (real violence), regardless of the resistance and friction.
Once I decided for sure that GC was for me (after attending the Nanuet class with John, following a few months of other classes and a few private lessons with Al), I dropped all my other training, as this was the only way to rid myself of patterns and habits that stopped me from perceiving and moving with REALITY. This was no small thing to me, as my Wing Tsun and Escrima teachers and fellow students had become like an extended family, and I got ex-communicated by some of Carl's guys for even suggesting that Perkins is legit. (Notably, the guy who originally introduced me to Carl, one of the longest-term and smartest of his students/friends, is now John's student.) After I devoted myself to Guided Chaos training exclusively, most of those movement habits and patterns that I had been forcing into myself for years dissipated within a few months. Why so quickly? Because they were not natural (no matter what my instructors had said), and once Guided Chaos gave me permission to yield to reality rather than blindly fight it, and then constantly exposed me to reality, my subconscious mind and body quickly ditched what they knew were useless habits. Of course, I'm still trying to break certain habits of mental and physical tension that may or may not be related to my previous training, but the overt stuff dissolved quickly. Now it's mostly a matter of improving my ability to perceive and act on reality with maximum accuracy and celerity.
I find it impossible to practice other martial arts while practicing true Guided Chaos, because while the point of Guided Chaos is to free your mind and body up to adapt spontaneously and efficiently to all violent motion, the point of other martial arts (besides Systema, which I commented on in Newsletter #90) is to restrict your mind and body to those motions, positions and ideas that the arts dictate are optimal for their limited paradigms of combat. The two endeavors actually work against each other! It IS possible to practice any martial art while also practicing the Guided Chaos exercises. However, simply practicing the exercises is not the same thing as practicing Guided Chaos.