Hi.
I agree that for breeding something in the 20% protein range is preferred. I use a flock raiser. I only add electrolytes and vitamins on occasion. However, I do ferment my feed which gives probiotics. Check the link in my signature line if you like. What made my final decision to continue fermenting was the difference in smell and consistency of the poo. That alone is worth it!
Chick starter as long as it's unmedicated should be fine as well. I avoid layer as I just don't feel like it has enough protein to fit my needs for my mixed age and gender flock. And the calcium in layer can cause kidney issues if given long term to roosters, chicks and molting hens. Also, this way I don't have to worry about feeding anybody separately. I ALWAYS provide oyster shell free choice on the side. And I use washed river sand (play ground sand is too small) in my coop plus free range so my birds get their own grit without me supplying it.
Scratch is like a candy bar and does zero to enhance your feed but rather likely actually diminishes your protein level. I avoid that as well. But if you are going to use, do so very sparingly.
The vitamins in the water probably won't hurt. But if they aren't needed they will pass through the bird and you will get some expensive poo.
The feed is formulated to meet all the needs of the birds without supplements. For me, I wouldn't do it full time.... but I don't have any illness and I have a hard time wasting money. If I thought it was needed though I wouldn't hesitate.
I have gotten shipped chicks that had many star gazers and feel as though better nutrition at the breeding facility would have made a difference. Once I discovered the issue I started dosing with vitamins and saved a few chicks.
Here is one link with useful info. Take note it says not to supplement for more than 10 days in a row. And it doesn't have specific info on breeding hens. But take what you can from it.
http://ucanr.edu/sites/poultry/files/186894.pdf
We all have to do what we think works for us. Some will argue layer is fine. And it is for laying. Many use only that for hatching too. That is not my preference especially if hatching with a broody who does not need the extra calcium. But we all have reasons we do what we do. And that's what's great about being able to share on BYC, the opportunity to learn a new, possibly better way.
My first test hatch... I have 18 out of 18 viable at day 12. I expected little success. I have to think some of it has to do with the strength of the flock laying flock.... but I don't know for sure.
And I know one person on here who's friends family runs a hatchery and uses layer. Says their birds are great. But there are lot's of things hatcheries do that I don't. Doesn't mean their way is wrong, just doesn't work for me.
Good luck!