People vary a lot (like a factor of 4) in their shakiness and cameras vary in the mirror weight. For sure you can shoot at slower speed with a rangefinder than a big SLR, and probably slower with a 645 than a 67. Having a long heavy lens on the camera will damp the vibrations significantly due to its moment of inertia.
With tripod and MLU, I try to go no slower than 1/15 on portraits just for subject-motion reasons. Your hit rate at that speed is not 100%, but there will be a good fraction of shots that are razor sharp.