Companion planting for cactus...that is an interesting concept.
I think it is probable that some cacti grow better and actually benefit from having certain plants near them.
When i did a bit of reading on Toumeya papyracantha, it was mentioned that this cactus is often seen growing in or near Blue Grama grass (Bouteloua gracilis).
I was wondering if the T. papyracantha cactus and the Blue Grama grass and maybe a fungus all work together to ensure healthy cactus roots and increased nutrient uptake from this rather finicky specimen.
I also recall reading that Aztekium ritteri is usually found near a type of lichen and that Mammillaria theresae grows in and among mossy patches.
It could very well be that certain species of cactus have a beneficial relationship with certain mosses, grasses, lichens, fungi and even other plants...but getting them into your cactus pot might be another thing.
dvg