Starter and temperature is the only way to insure, plus heating you milk first. Its amazing how much bacteria is already in otherwise fresh milk.
Let take the Acidophilus/Bifidus yogurt starter, it needs to start at ~70F and slowly rise to allow different bacteria in the mix to populate. Apparently once a certain strain starts working another is denied access so for this batch its important to start of with low temperature.
The other strain they sell is the bulgarian yogurt which isn't as solid but for a lot a of people it is "yogurt" period no other strain matches it. It requires you hold the milk above a certain temperature without allowing it to cool first.
As for Yogurt vs. Kefir, Kefir is disallowed in SCDiet since it still leaves most lactose untouched, while a 24 hour yogurt fermentation will digest all.
Now lets be clear, there will be less beneficial bacteria in a 24 fermentation than a 15 hour fermentation, that is because the rise in PH kills some.