David - The diascias (and many other tender perennials like salvias and so on) rooted quite quickly in that mixture, so there was no need to feed at all. As soon as roots were growing, I'd lift the cuttings and plant up in pots of compost, in much the same way some people root cuttings in water really, except the roots formed with resistance of sand and vermiculite were sturdier and grew on better. But I have occasionally rooted just in water and it has worked as well for diascias.
Hans - yes, a few things might rot, but mostly, if the mix is kept moist but not wet, root very well. I just throw out failures, but there have not been many really. The other thing I usually do is keep the container out of any direct sun during the process, which takes just a few weeks, often just two....
