I have what I looks like a Euphorbia, but online the photo is identified as either Kleinia stapeliiformis or Senecio stapeliiformis. Common name is Pickle Plant. Which genus is correct and how much like a Euphorbia do these behave?

I have this planted in 1/8 inch pumice, which is moist but not water-logged. The plant is starting to rot in several places, so I assume like many Euphorbia it wants a very dry soil. What would be a good soil mix for this species, left outdoors in zone 9b in a pot? Winters will usually be above 40F at night but can tease down to just above freezing.

Should I go 3/8", 1/4", or 5/16" for the size of the crushed gravel?

Should I avoid any organic component?

Would it be helpful to add in turface here?