From what I understand, the Cornx chicks develop leg problems due to two things.. faster than normal growth, and extra weight (muscle) in combo...
From the way you are talking, your chicks have neither of these problems... So personally, I would question (and label) the causual agent of the leg problems as your target to the source of the problems you have experinced with these birds.
Maybe it's just me though, and my own uneducated opinion..