For sexing purposes, "cuckoo" and "barred" are the same as each other.
The head-spot sexing should work for both.
The males being lighter than females should also work for both.
BUT both of those points only work in birds that are pure for the barring gene. If someone crosses a barred female...
No, Australorps do not have feathers on their feet.
I would guess you have a Marans-mix or a poor quality Marans ("poor quality" meaning it doesn't look quite right.)
Sex-linked Marans can exist.
If someone crosses a Cuckoo Marans hen (has white barring) with a Black Copper Marans rooster (no white barring), they will get sexlinked chicks. The sons will have white barring, and the daughters will not.
It works for any pairing of mother with white barring...