Kefir contains S. Cerevisiae (ref), so i don’t imagine that it will conflict with your supplementation with baker’s yeast. Still, though, it might be a good idea to keep them separate if possible. My ND told me this is a good rule of any differing probiotic supplementation.
I do remember Jorge saying not to use S. Boulardii and Kefir together as they may conflict. I haven’t seen any scientific articles that provides a view one way or another. However, I believe organic, homemade kefir is such a useful tool that if it does conflict with S. Boulardii then you’re better of getting rid of the latter rather than the former. I found this especially true as you might have a die-off reaction if you include, for instance, turmeric or peppermint in your diet. When I was using S. Boulardii there were just too many little potential conflicts. I decided in the end that I’d rather have everything working together rather than sometimes against each other.