Medic:
1. I can't find any evidence for or against fitment on that Surefire mount. If it fits, it's likely just a coincidence, we did not design it specifically to be compatible with their mounting systems. We do know that the Haley Strategic Thornail MIL620 mount will work, but that's also a happy coincidence, neither product was designed with the other in mind. The measurement from center screw to center screw is 1.782" or 45.26mm, if you want to compare to other mount specifications you find online. I wasn't able to easily find the spec for the Surefire unit.
2. I don't think I understand your question with the information available. Can you attach a photo of the mount you are using on your Holosun? And what mount are you considering using for the magnifier? Is this an AR application or something else?