might be great advice if you happen to be studying at the One True School™
As far as I can see, I estimate that the “don’t mix practices” advice must abused significantly more often than used, and to comercial or power-play conveniences.
There’s what I find to be a fundamental point here, that iamthatis referred to, which is the danger of being an “etheric body mechanic” - that is, the danger of trying to tweak with aspects of ourselves we have little to no understanding of.
To give a bit of an extreme example for the sake of illustration. I’ve heard of different forms of acute yoga that burn calories with almost extreme intensity, and the practitioner must eat enormous amounts of pure fat and will still lose weight. Other practices involve very long fasting and almost negligeable amounts of food for extended amounts of period, years even.
I’m quite confident that if the calorie-burning sadhana practitioner forewent food, he might just die in a few days. Also, if the calorie-preserver sadhana practitioner ate buckets of butter, he might get very, very ill.
Of course, few of us do any practice with a fraction of such intensity, and most likely doing some mixing won’t be fatal or have noticeable effects. What I do know is that I’m very ignorant of the fundamental mechanics that powerful sadhana touches. And, similarly as I’d expect from experimenting with different powerfuk chemicals, mixing different forms of sadhana may eventually have detrimental effects (and of course, possibly quite benefital ones, too). Even if I can’t tell exactly why I experienced what I did, I find it likely it has to do with some mixing. And I was being quite cautious.
That is, I mentioned schools and maybe that was a misleading term. I’m not referring to any book, dogma, doctrine or group. I’m referring to the fundamental mechanics that makes us function, and of that I’m about as ignorant as it gets - demonstrably so.
And then, I see similarly as you, that no school can possibly contain all truth. That’d be similar to saying that some library contains all books ever. And in terms of finding truth, especially on the level of understanding, in my experience crossing traditions can be very useful if done with minimal discernment.