The yeast of kefir doesn’t promote a Candida infestation. If there’s a problem caused by milk kefir, it would probably be because the kefir wasn’t fermented long enough to remove all of the sugar before it was shipped. You can take it during stage 1 for sure. Also at least part of the cultures will be alive even though it’s pasteurized.
You can buy the grains and make it yourself which would be the best. If you decide to do this, use 1% Organic Valley Milk, that is if you can get that brand, if not, any brand would work.
Look for the Rumford brand of baking powder which comes in a red container with white letters. It has the words “Aluminum-Free” printed across the front of the container.
I wouldn’t drink buttermilk during any stage of the diet.