Production reds do not have white on tails or necks(they basically look like a lighter colored RIR). Red sex links do.
if some of the 'sex links' grow up to be roosters then very likely they are from mixed flock of production red and red sex links.
The white has nothing to do with sex linkage so it will not indicate sex in those birds. This is because sex linked matings work 'only once' and in a very specific way.
But agree the amount of white can possibly indicate sex, because they have dominant white, which changes black to white yet leave red mostly alone. Roos have more black on their necks so- more white on neck- possible roo.
but if they are mixed up it won't be a hard n' fast rule- that one you wondered about brown leghorn is probably a result of mixing and she did not get the dominant white so she looks 'funny' to you but normal for not exactly purebred mix.
Go by pointy feathers, larger combs than their buddies etc. I see some possible boys and girls in the group pic.
if was not clear enough, all of these are likely production red mixed with red sex links, including that 'mystery chick'. She just didn't get the gene for changing black into white. She will grow up to look a lot like a production red.
also to suspect the EXTRA lighter birds being boys in this batch. The ones with more white on neck. back, tail area. In a few more weeks the sex will be much clearer, boys starting to get larger also combs getting thicker and larger on boys.