I honestly don't think that feeding kefir or kombucha will prevent coccidiosis. That is caused by a parasitic protozoa that feeds off thiamin inside the birds. Medicated feed contains a thiamin blocker that will prevent a build up of thiamin that the coccidia need. If you don't plan on feeding medicated feed, you need to have medication on hand to treat for coccidia. Even delaying treatment for a day can kill a chick. Since Kefir contains a lot of thiamin, I think it might encourage coccidia instead of preventing it.