Your peep will just need to be kept inside abit longer, since it's so slow to feather out. With a friend of course. Feed isn't the issue, it's just a genetic trait thing happening.
Is it, 18%, or 20% protein?
You don't have to change your feed if you don't want too. I personally feed 24% unmedicated starter/grower for all my chicks. I also mix in a Ration called Farmer's Helper Ultrakibble, that's close to 30% protein every once in awhile(Maybe a week, or two apart each...
She's losing fluff faster then it can be replaced with juvenile plumage. Also seems slow to feather out.(Trait, not an issue)
What protein content are you feeding?