I assume that you mean to use after the MC as you should not be taking it on the cleanse as that would stimulate the digestive system and work against the main operating mode of ANY fast which is to give the digestive system a rest.
Kefir is excellent and my favorite. It is superior to yoghurt in that it has like 8 beneficial bacterial strains to yoghurts one. I addition the bacterial strain in yoghurt is transient and won't colonize in the gut as will the strains in Kefir. Kefir is also less mucous forming. It can also be made with non dairy products like coconut milk.