B737-800 pilot reported push back crew did not follow SOP and aircraft rolled backwards before flight crew applied brakes.

Narrative

Push crew had no working headsets. Push crew did not make contact with flight crew prior to push to review safety or push signals. Push crew did not know correct signals or engine start sequence. Push crew never gave the 'set brakes' signal. Tug driver disconnected tow bar without giving Captain (CA) the signal to set brakes. Aircraft (A/C) began to roll into tug as tug tried to back away at same speed. I kept looking at guideman anticipating a 'set brakes' signal but none was given. It appeared that the tug was pulling the A/C back in an attempt to straighten it. What was actually happening was that A/C was disconnected and was moving forward at the same speed tug driver was trying to get away from the A/C. My concern was that if I used brakes to stop the A/C that I might damage or break the tow bar - if in fact we were still connected. I made a decision; based on the panicked look on the guideman's face; to stop the A/C movement with my brakes. Push crew not using SOPs. Better training and adherence to SOPs by push crew.