Mine have been thriving, growing, and pumping out babies for months in a tank with kH 4 and pH 6.6. They even made it through a small pH plunge to 6.0 along the way. Although they haven't made it past the year point, when I usually declare success, they are simply thriving.
Up to now, I have always had them in alkaline conditions, but I wanted someone to eat hair algae in a planted tank. The more I read, the more I wonder about the whole high pH/high kH issue with platies. They live in weedy ditches in the wild, which presumably develop low pH as well. As with most fish, the specific pH is probably a lot less important than clean water and stable conditions.