Orpingtons are a slow developing breed. So its normal for them to mature to size slower and feather in slower. The dark headed one is just an odd colored Barred Rock/Cuckoo Marans. She may change color after her first molt.
As for as Barred Rock or Cuckoo Marans check the legs. White leg =Marans, yellow legs = Rock. They all look to be pullets. Barred/Cuckoo cockerels would be much lighter in pattern as they inherit two barring genes.