Can I use this shredder blade with an FS260? If so, what else do I need to get.

I am very sorry this blade is not listed for your machine. I have checked and we have this blade listed as suitable: https://www.lsengineers.co.uk/brush-knife-300-3-12-3-for-stihl-brushknife300-3-part-no-4119-713-4100.html and we also do a replacement version: https://www.lsengineers.co.uk/3-blade-metal-brush-knife-300mm-20mm-hole.html This what you would need to attach it: https://www.lsengineers.co.uk/large-metal-blade-fixing-kit-for-stihl-brushcutters.html