Your white chick doesn't look like a cornish cross to me. She does look kind of like my white leghorns did. Were leghorns and cornish crosses the only type of white chickens your hatchery was selling? If you could take a picture of her alongside one of the other chicks (for size comparison) that might help us to identify her.
As far as the hatchery selling only 1 type of black sexlink;
Since the chicks are a mixed breed, they will not be exactly the same. I also bought all our black star chicks from the same hatchery and there is still a huge variation. They all have the same body type and look about them, but the coloring differences are large.
I went out this morning and took pictures of my three remaining 17 week old black sex-links (sold the rest because we had too many).
This pullet, Amber, has a lot of gold on her chest and neck:
This one has some brown on her neck and brown scattered lightly around her body feathers (the chicken in the background is not a black star):
This one, Ebony, is almost all black - she has a little brown on her chest.
All 3 of them together with a frontal view of Amber.
Besides the brown in your chick's chest, I can also see a little brown on her wingtips - I think she is a black star, just like mine. We love ours - they are friendly - have always let us pick them right up without trying to run. They are not laying yet, but should be excellent layers. They are also pretty birds and easy to distinguish from each other because of the unique variations in color.