It depends. A good martial art should be a system where everything works together based on the same mechanical and strategic principles. Different martial arts have different principles. Mixing and matching styles can be like mixing and matching operating systems for your computer, which generally doesn't work out that well.
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It's not about "purity," it's about everything fitting together such that the whole is greater than the sum of it's parts.
Agreed.
IMO, there’s nothing wrong about learning some things from different styles, or learning about styles. But you need a very strong focus. If you look at different Martial Artists, MMA fighters and even boxers, they have very clear styles. Some are dancers, bouncers, sluggers, etc. They don’t really change between different modes. They are what they are. The problem with different Chinese arts is that they have very different modes and mind-sets. Techniques are created for certain purposes. Long Fist kicks are designed for Long Fist strategies and tactics, Xingyi punches are designed for Xingyi tactics. The strategies of these arts are quite different.
So, IMO, it’s better to not just mix, but to first find out what kind of fighter you are and want to be. Then you can work from a core and build different methods and techniques around this core in a way that makes everything match together.
My first art I studied was Tai Chi and it has always been my main style though I have practiced some other things like Sanda, Thai Boxing and Shaolin. I don’t find arts with a very strong external focus compatible with the internal arts. Bagua and Xingyi is a more natural combination as they have similar focus and approach and they work from similar angles and distances. I have said this before, but I studied both Bagua and Xingyi for many years but decided to throw them away as I wanted to spend my practicing time on Tai Chi only. I am completely satisfied with Tai Chi Chuan and have total confidence in this art. I see no real benefit about combining this and that from different arts. It can be fun and in some ways rewarding to combine arts that are compatible though. But I also think some people miss what is the most important and hide themselves behind techniques. Again, you must find a way to approach fighting that suites yourself and that is true to your heart. Otherwise your whole approach to martial arts will be confused and unfocused.