Bob has made some interesting observations regarding this, but a yellow will never have the same shiny coat as a chocolate or black just because of physics. I do remember Bob posting about his morning walks with Puff and how he can detect a shine in the right sunlight.
As for the age, I don't know when you can start. I've stopped because the current kibble Jes is on does the job without supplements. I believe he was around 1 year old when I tried feeding fish once or twice a week, but that doesn't mean you have to wait that long.
With all the stomach issues you dog has had, do you think it's a good idea to start something new? Personally, I don't believe in adding all sorts of stuff to a puppy's diet. If you feed a balanced kibble, there really is not a need to supplement.