That is one reason I use flock raiser instead of layer as well. I don't want my longer term roos to *possibly* have kidney issues caused by the extra calcium.
Many feed their rooster and hens (even chicks) layer for their whole life and still live 8-9 years. But knowing too much, I prefer the oyster shell on the side. In addition, 16% protein is the MINIMUM needed to sustain a LIGHT bodied hen in good condition. Most mine are not light bodied but dual purpose and I don't go for minimum nutrition, which can especially be effected by people who think they are giving their birds "good" lettuce or fruits and veggies.. not realizing that the feed was balanced and what they just added had little to no nutritional value and diminished their protein level. It's the amino acids in the protein that are important (and some of which need to come from an animal source but substituted as an added nutrient similar to cereal vitamins and minerals). And feathers are made up of 90% protein, being one reason why show breeders often go with higher (28%) in order to get healthier, shinier looking feathers. To me there is also a point of too much protein, which could also cause kidney issues but for different reasons than too much calcium. Balance is definitely key.
Also, noting the word Muscovy in your user name... when I got ducks I contacted Purina to find out what their level of niacin was since it isn't listed on the bag and that is a KEY nutrient for ducks. The level is 55 ppm, which is the correct amount for ducks to not develop leg issues due to deficiency. I wish they didn't (recently decided to) include DE, which is approved by the FDA at up to 2% as an anti caking agent. But alas I have not found any PERFECT feed with regards to nutrients, ingredients, and cost. I'm a huge label reader and spent lot's of time last year exploring my options. For ME the flock raiser is the best (not perfect) choice. I guess, often having chicks is probably another reason I will stick with the crumble. But I do buy pellets and throw those out as "scratch" or a treat on occasion. They get excited about it but NO diminishing of nutrients!