Breed standards says they shouldn't but that was the closest matching description that I could find. You could try posting it over in the Breed/Gender section and see what they say. The people over there usually have more experience. Or you could wait and see what happens when the chick gets older. Is there any possibility that the 5th toe is a spur?
Eeeks, now you got my curiosity up. I just got 16 standard blue cochins, 2 months old, I have to go and count how many toes they have, always an adventure in wonderland. Have a great day, CynthiaM.