Setup Motion Sensors to Trigger Cameras only

No.  In fact, the cameras aren't connected to the system, so if they trigger via their own motion sensor they don't activate the alarm.  

If the alarm goes off, then the cameras all start recording.

As mentioned, the cameras have their own motions sensors, so you may not have to do anything to get triggered by motion.

But if you want to, you can set a regular motion sensor to "secret alarm" and when it detects motion, the camera(s) will record.