I had decided a year ago to explore soil the chams could eat and wouldn't harm them. What you have to realize is all this "organic" soil have manure in it and many times chicken manure. This is exactly what we want to avoid.
After searching for months I finally found a cactus mix that contained no manure what-so-ever. No worm issues and they've been eating it for a while. Plus I had to repot a Alii Ficus tree and the nursery even recommended using a cactus mix. Tree and and Chams are fine.