Medicare will not pay for therapy codes done under a chiropractor.
Most commercial carriers do pay for therapy, but you don't need a modifier.
If billing physical therapy codes to Medicare under a MD, DO, or physical therapist, you will need the GP modifier. Also, you will need to reference the crosswalk to ensure the DX codes are pointed to the right procedures. Your local Medicare contractor's websute should have a copy of that for you.
Yes, you will need to indicate the AT modifier on any adjustment codes if the treatment is for an Acute or Chronic Injury. Medicare does not cover maintenance adjustments.
The GA modifier is Medicare specific. It indicates that there is a ABN on file for that patient. So you do not use that modifier for commercial carriers.