If you study food therapy, you may learn that it is better not to eat spinach and Tofu (beans) together, both of them are super nutritious and beneficial for the body, why can't they be mixed? It is because that spinach is high in a substance called oxalate (H2C2O4), when it mixed with calcium(Ca) in Tofu (beans), it will becomes Calcium Oxalate (CaC2O4), so your body can not absorb the necessary calcium for its needs and when intake too much of both, it can cause kidney stones.
China is the original birth place of Qigong, all the authentic and sincere Qigong masters emphasis that not to mix different Qigong styles, and students take on that as a common sense. There is few "risk-taking" students will try to mix, unless they mix bunch of simple physical exercises, it will not feel that harmful but usually they may not make much progress either; If one mixes two real powerful and pure lineage Qigong styles, one for sure will feel disoriented and scattered of Qi right afterwards; if one mixes two practices for a long term, the result will be very clear, one can not compare to the benefits and progress of students who focus on only one style.
Even though, Chinese Qigong masters agree upon "practice one style of Qigong at a period of time", they do respect other Qigong styles and other Masters' work.
It is like climbing mountains, there are many different trails, but eventually when we get to the top, the view is the same. People have free choices to try different paths, but It can also be a waste of time and energy to switch from one path to another frequently, when one still could not decide which path to take and spend energy on running back and forth, other students might have reached the top of the mountain already.
Thus, when you find the right practice, keep doing it. Except at the beginning or once a while cleansing process, if you become healthier, stronger, more peaceful inside and greater progress of Qi state, you know you have chosen the right practice and you have been practicing correctly.