Hi, all. I am curious about the soil that is best for rooting these plant cuttings. Not leaves, but tops.
Also, if these plants have long stems, what do you do?

I use the same soil I prefer for growing these plants --- about half pumice, but you can substitute perlite, lava rock, aquarium gravel, etc. and get similar results. It depends on the plant (any chance of photos?) but I usually trim long stems back to half an inch or so on cuttings. Partly to avoid burying too much stem (something to avoid with succulents in general) and partly because a lot of rosette succulents like Echeverias and Aeoniums (not so much aloes and their kin) actually root faster and better from short stems.