The CS specification on the SM160 says it can do 20 whole chickens at one time. So if you assume about 2 kg per chicken, that is 40 kg of chicken at one time. So it seems that the SM 160 should easily do 20kg of chicken. Your lowest outside temperature of 6 C (about 43 F) should not be a problem.
The only suggestion I have is to make sure your product is dry of marinade before you place it in the smoker. Much of the smoker heat will go to evaporating the marinade before the smoker temperature will get high enough. I hope this helps. Good luck.