If you are using 2 different plans - one for supply planning and other for ATP, why would you want planned orders created in ATP plan ? Planned Orders are suggested supplies which in my opinion should not be used for ATP. The planned orders from supply plan which are firmed and/or released will anyways come into the ATP Plan after data collection in form of new PRs, POs or WIP Jobs. For ASCP & GOP implementations where I've worked so far, I've turned off planned order creation in ATP Plan. Firming the POs in data collection staging tables using a custom script is a good solution to avoid cancellation exception. Similar approach can be used for work orders to avoid reschedule-in exception.