Yeah, I would highly suggest some of you get a book (preferably something written in the last 25 years because some of the information/advice posted on these boards lately is ancient). Chick starter is fine for waterfowl. We go over this about 6 times a day here anymore, LOL. Most medicated feeds are fine for waterfowl these days as most are medicated with Amprol, something that has been known to be safe for waterfowl for at least a couple *decades*.
All that said, we do not feed our waterfowl chick starter. If that is a person's only option or if they prefer it, they certainly can though. In about 25 years of raising waterfowl though, I have never lost even a single bird to cocci. I, personally, also do not like feeds with a lot of protein. We are currently using Flock Raiser (20% protein) for the first couple weeks with oats sprinkled on top from day one. Starting within the first few days of life, we add more and more oats until it is approximately 25% total of the mix (to bring down the protein). We also add fresh greens and Brewer's yeast from the first week.
The OP's problems (at least the ones I can comment on) were probably excess calcium and lack of Niacin. It has LONG been established that excess calcium will kill baby waterfowl dead as a door nail. Layer feed for goslings is an awful idea (as is exposing them to a predator). A lot of the other information in this post is subjective. Different people may offer slightly different things, for example some do feed a slightly higher protein and their birds are fine especially if they can forage. Some of it is a matter of fact and some is opinion.