Is the yeast/bacteria balance in mature sourdough starter (fed on wheat and barley) correct for making kefir? If not, why?
No, you cannot. Kefir grains are a unique Symbiotic Culture of Bacteria and Yeast (SCOBY). Kefir grains are a gelatinous mass of microorganisms including Lactobacilli, Leuconostic, Acetobacter, and Saccharomyces. By looks, it is more like Ginger Beer Plant than sourdough starter.
It is not possible to create kefir grains from scratch. You will have to buy a starter, or get a donation from someone you know.