The Sealife DC1400 comes with the flash link adapter (that circular piece you mentioned). The 1400 model is close to the 1200 so I would call Sealife and check if the adapter would fit on the DC1200 or not. For cameras that doesn’t have the flash link adapter, the flash link cable you purchase usually comes with an adhesive pad that you can attach the cable and glue it onto the camera housing at the flash port. And yes, you can use any strobe that accept flash link cable. I’m using the DC1400 with an Intova ISS 4000 strobe and they work great.
Hope that helps :)