I am most familiar with Tibetan Buddhism, and the lamas also say not to "mix" practices. It's the same in all the esoteric traditions, and there are good reasons for it. You CAN do several different energy practices from different lineages, as long as you space them out properly and separate them. But you wouldn't mix them up in the same practice session, or mix them up together as one practice. Even the Dalai Lama doesn't do that.