Like most candy with a coating there is some degree of processing, drying time and technics. Anhydrous would be more preferable because its moisture free. The less moisture on a coating of any sweet the better. Citric acid just like sugar attracts moisture.
I've had success with the Sour Patch Kids recipe from Classic Snacks Made from Scratch: 70 Homemade Versions of Your Favorite Brand-Name Treats. The quantities are merely an indication for the coating recipe. Just see how much you used and then subtract or multiply the recipe accordingly.
• 2 T powdered sugar for dusting
• 2 T cornstarch
• 2 T granulated sugar
• 1 t citric acid (anhydrous)
Whisk the powdered sugar and cornstarch together in a small bowl, and whisk the granulated sugar and citric acid together in another small bowl. Set aside.
Set a wire cooling rack in a rimmed baking sheet, making sure the rack fits comfortably inside the "walls" of the sheet.
Lightly dust a cutting board with powdered sugar, spreading it with your hand to make an even dusting. The moment your gummies are ready take them out of the pan you had them in. Flip over each gummy once on cutting board so that both sides have a fine coating of sugar.
If the candies are starting to "weep" and get goopy and sticky first dredge them in the cornstarch-powdered sugar mixture, a few at a time, tapping on the side of the bowl to remove excess powder. Then toss them in the sugar-citric acid mixture. If the candies are dry to the touch, simply coat them in the citric acid mixture.
Let the coated candies dry for 8 hours on the cooling rack until the coating is hard and crunchy.
Store the candies at room temperature in an airtight container for up to a week.