OC+ would work bangin' well in Promix.
Coco....I'm not sure. Coco does some weird Ca/mg binding thing, I'm told....I just don't know if it would work or not.
I was over at the site where the alleged real Hempy hangs out, and he has gone to using coir. Is that the same as coco? There's also a pretty pissed off debate going on there about the amount of runoff. Some say none, some say some, and some say 30%. I can't see that Hempy actually says anything about it, well, at least that I can understand But he did mention that he has gotten something like 24oz off a single bucket
TA is what gets us into trouble, especially if the pH is also high. Even though you add pH down to get it into range, the bicarbonates build up and raise the pH of the medium, in spite of the pH down in the irrigate. On top of that, you get way more P than you need from all the phosphoric acid.
On the other hand, if the TA is low, it matters little what the pH is, because the buffers in the soil or the nutes will easily move the water pH where it needs to be.
TA of 40 is great. Under 80 is fine....but when it gets up that high there is no need for cal/mag.....certainly no need for Ca.