The article recommended soybean, canola, and cotton
seed meal as garden amendments. Not sure about cotton seed but soymeal and canola meal are very commonly included in poultry feeds as a protein source. In garden soil feather meal would be a slow release source of nitrogen. Feather is mostly composed of keratin, a protein, it could be fed to chickens.
Too much fish meal fed to chickens makes the eggs and meat smell and taste fishy, in limited amounts it is a source of protein. If you do not mind the taste in your eggs it would be worth experimenting with if the price is right. It would be a source of nitrogen and trace minerals for garden soil.
Phosphorus and calcium need to be kept in the proper balance in living animals. Rock phosphate is a source of phosphorus. Gypsum is a source of calcium. Bone meal is a source for both. I don't know how well a
chicken can utilize any of these sources. You could try it if you want. Assuming you have layers, watch the shell thickness and texture. If you have problems then you have problems with the calcium levels biologically available to your chickens.
Kelp and azomite would be good to offer for trace minerals.
Chickens will eat what they need as long as it is available. If their manure never leaves your
yard than the minerals will slowly be cycled into your soil.