I would not recommend using reduced-fat or fat-free yogurt in this recipe because it will not set up the same, as it has more liquid and a thinner texture. Whole milk Greek yogurt is very thick which helps with the texture of your filling, keeping it creamy and light. Sometimes even whole milk Greek yogurt can have a fair amount of liquid, and if this happens you can strain it in a few layers of cheesecloth to remove that extra moisture.