Although you're not responding to me, I found your post very helpful. I have five baby chickens (four were hatched on April 9, the fifth was hatched on April 13). OF the four hatched on April 9, two are Golden Buffs (Red Stars). One GB is growing well: she's feathering out along with two of her sisters hatched on the same date. The second GB is lively: eating, drinking, pecking, and sleeping quite normally when compared to her sisters. And no one is pecking at her. But, she is smaller than the rest of the three hatched on the same date and feathering quite slowly. The fifth baby, hatched on April 13, is beginning to surpass her in growth. Because the "runt" is a GB, a sex-linked breed, I don't think she would end up being a roo. I've read that roos can grow more slowly. I'm concerned about the "runt's" slow growth, but it seems like your experience is saying that she could just be small and she'll be fine; or she may not make it for whatever reason.
Is there anything I can do to encourage her growth? Mealworms? Protein? They get water with electrolytes and probiotics in it, and they're eating a starter/grower feed with 18% protein.