You can start with a lower dosage and some docs do that, but you should aware that the actual beneficial effext of imuran (azathioprine) only takes effect once you reach the dosage in the effective range (usually 2mg-2.5mg per kg bodyweight). Some people only tolerate lower dosages, in so far as their leukocyte count is at the lower end of the normal range or even below at a dosage below 2mg per kg bodyweight.
Or in other words, there is no reason you can't start with a lower doage, but that means the beneficia effects of aza will take even longer to kick in as they usually do (usually up to 3 months).