Some plants don't need a lot of roots, in fact I have a lot in my garden that just really have roots to hold them in place, and the roots hardly take up much nutrients at all.
Your Jade plant takes up and stores a lot of nutrients in its fleshy leaves. As long as it's got healthy looking leaves, it will grow the roots it needs. I think the Cactus Mix will stimulate it to grow the right amount of roots and they will stay healthier in the loose, airy medium.
