So long as they are both V1 they are compatible. The only issue is spindle length. FC-6500 takes 109.5 mm for a 43.5 chainline. Sheldon's site says M952 is 112-116 mm for 47-50 chainline. Using the crank on the longer spindle isn't terrible. How good or bad depends on the rest of the bike. What's the rear chainline? And also consider chainstay clearance if you want a shorter spindle.