M. plumosa and soils

I have 2 M. plumosa which I understand can rot off their roots if overwatered. I am a heavy handed waterer, and I am wondering if a course draining mix will serve plumosa better around here. I use 2, straight pumice and Al's Gritty Mix or a variant thereof. My gritty mix is 1/3 pumice, 1/3 chicken grit, and 1/3 fine wood chips.

You're right that it can rot if the roots are kept too wet. It can be quite sensitive.
I'm sure that straight pumice would be OK, especially if it isn't too fine. But I would worry a bit about the wood chips, as if they are fine they might bind together and clog up free drainage, which is what I believe M. plumose likes.
I have seen it in habitat, growing on steep rocky slopes in Huasteca Canyon near Monterrey, NL, which is mostly limestone rocks, in cracks where it can get its fleshy roots into accumulated grit and humus. This says to me it likes free draining mixes.

Pumice it is. Thank you.