A lot of times the people handling the chicks at feed stores don't know a lot about chicks or chickens. They may come in segregated but they can easily be mixed up. At my
Tractor Supply, they have three tubs, red, bantam, and others. They may get red sex links in separately but in mine, they get mixed up real quickly with any other red chicks.
The barred gene is a sex linked gene. Females only have one barred gene while males have two. This makes the males usually lighter in color than the females. If you have a bin of only barred rock chicks, this works pretty well, but if you have a bin of mixed chicks, some of which are barred rocks and some that are something else, your certainty goes way down. You could pick out a darker barred chick and select a black sex link male, for example.
Yes, sex links can make as good a pet as any other breed.
If they have them separated, the red sex link pullets are reddish while the males are yellow. With black sex links, the males have a white spot on their head that the female does not have. But if they don't have them separated, well, good luck!