I know that you can expect great accuracy with bullets as light as 120 grains....but I've no experience with anything lighter than that. There are more than a few 6.5 x 55 owners out there, but the owners of .260's should be able to give you relable info that will transfer to the 6.5 x 55 adequately. I guess if you have some of the lighter bullets on your bench I'd suggest filling 20 or so cases with a medium speed load and just try them at the range. I know that the latest LEE reloading book gives loads for 70, 77, 80 and 85 grain bullets, Sierra gives loads for a 85 grain bullet, the NRA publication titled "Handloading" gives loads for 77, 85, and 87 grain loads
However, my (old) Speer starts with 120 grainers and my (old) Hornady manual starts with 100 grainers. Good luck......My guess is that you'll get acceptable accuracy down to even the 87 grain balls unless you crank the speed up to the upper end of the suggested loads.