I am a novice, but I asked Mike Lucas - Lucas Bore Guides and he said the .22 to .26 should be fine for 6.8. The length of rod I ordered was 36" which should be Ok, because the bore guide is about 8" long or so.
With respect to the bore guide, you can buy one for 5.56 and one for 6.8, and only buy one insert for the rod that will work in both bore guides. That will save you a bit.
Again, you will want to get some expert opinion, like I was trying to do. I sent the rod back. I shouldn't have to deal with a rod that was curved from the start.