Just plain potting soil, Lynn. It is flexible so soil used should be appropriate for the companion plants which likely will be more fussy. I counted six plants in this pot and the other pot is similar. They will "tell" you when they are thirsty-- the leaves will get a little leathery looking. I can't tell you how many of these I have composted every time I divide. Then at one of our garden club's seed/plant exchanges this one member brought about 30 separate divisions and refused to take the extras home so I was stuck with those.
In that Timber Press book about growing on tables and flat surface there is a cool photo of the tall type Sanseveria growing on a slab of granite with the shorter birds nest in front and maybe some type of sedum(?) also. The whole thing is backed to a stone wall. (page 176)
