I am looking into making icing using yogurt powder. My question is, would it still need to be refrigerated after I make it. The ingredients in the product I am look at are nonfat milk solids, whey solids, lactic acid.
It will absolutely need refrigeration. The reason why the powder is shelf-stable is that it is dehydrated and lacks the water bacteria need to survive. Pretty much everything you could mix it with to make an icing will contain water, meaning that it is no longer shelf stable.
Even if you were to mix it with something without water, e.g. shortening, the strict food safety answer is that it still needs refrigeration until somebody has ran the necessary microbiological tests to prove that it doesn't.