Six centuries would be 6 units of 80 men totalling 480 men. So actually it's quite easy to make a cohort. Take 6 units(on large size) and the group them together, march em together fight em to together and if need be you can split the cohort up to handle multiple threats.

Did the cohort historically fight in 1 big group or 6 different groups but within a very close proximity to each other.