Definitely a puzzle!
Among the breeds you mentioned, some will definitely have thicker legs and larger breasts than others (like Plymouth Rocks vs. Polish), but I don't know exactly which ones are likely to be mixed for the mystery chick.
I wonder if the breeder has any kind of meat birds, and one of them got mixed in?
There are a few breeds where even the females have big breasts and thick legs. For example, purebred Cornish chickens:
https://www.feathersite.com/Poultry/CGA/Cornish/BRKCornish.html
Based on the color of the...
I agree, that would make sense.
I'm pretty sure it is barred silver columbian (like Delawares), but that same combination of traits can come from various mixes as well.
Male vs. female: you might be right, but it's not obvious enough for me to be sure yet.
Feathers on neck: to me, it is looking like Delaware feathering. The color pattern is almost the same, but Delawares have white bars across the feathers and Light Sussex do not. So on Delawares the black...
Ooh, one of those tricky ones.
Chickens with the genes for white legs can have light yellow ones sometimes, partly caused by what they have been eating. And chickens with the genes for yellow legs can definitely have light yellow legs or even white ones, caused by a different selection of...
For gender, I do not think the breed change will make any difference.
I am not sure that chick is a Light Sussex or a Splash Barnevelder. The patterns of colors in the feathers look wrong to me for both options, but I don't think there is any way to know for certain yet. Many chicks get...