Variety is best. Damsels are omnivores, and should have both meats and veggies in the diet. There are a large variety of flake, freze dried, and frozen products at your LFS
that will work, or you can make your own frozen food alot cheaper.
If you do buy your own, stick to buying the smaller packages, so everything stays fresh while you're using it, and you can more quickly rotate in more types of foods. Try to stick to items of saltwater origin, as they have diffrent ammino acids than freshwater foods.