Use the KISS (Keep it simple stupid) principle. Have you ever even done the MC before? How much do you know about fasting?? or how the MC works?? You have no idea how any of that will interact with the cleansing properties of the MC, not only affecting the cleansing but very likely keeping you hungry throughout the fast and possibly even make you sick, after all you are adding an ACTIVE bacterial culture when your system is trying to rest and cleanse, Geez Louise!! The MC has been around for more that 60 years and proven an effective effcient method AS IS. Why in the world would you want to mess with something that is a proven protocol when my guess is you've never done the MC before and possible any real form of fasting period. In all the years I've been around this forum the ONLY times people have had serious problems is when they have strayed from the true and tried original protocol. You feelin lucky........... If it ain't broke don't fix it!! Save the Kefir for AFTER the MC.
You want to experiment? Knock yourself out. You just won't be doing the Master Cleanse.