Use what you have on hand. Once it's in the pot, you won't see it and the Cymb won't care. With the caveat that when you have to divide it, LECA, scoria, etc, can really chew up the Sawzall blades quickly and dull your axe.
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PS For anything you choose to grow in a bark mix, Orchiata is really hard to beat. Buy in bulk though, not specialty type places.