The Swede performs best with slow burning powders, which makes barrel length helpful in getting the most out of the cartridge. I recommend 26" because I consider it the maximum practical length taking into consideration the barrel profile needed to ensure a reasonably rigid (read consistent) barrel, overall rifle weight, and balance.
I have a Swede with a #4 Shilen and a Swede with a #2 Shilen, both 26" in length. I haven't shot the lighter of the two yet, so I cannot evaluate it for accuracy. The #4 is lighter and easier to carry than my Remington 700VS in .308. It is accurate, delivering consistent .5 MOA three shot groups to 300 yards (the furthest distance I have shot with this rifle so far). The #2 feels nicely balanced and I expect it to be very pleasant to carry afield. If it yields the same accuracy as the #4, I would consider it an ideal carry rifle.
I don't think that a 24" barrel would represent a serious disadvantage in a Swede, but I suspect that the performance penalty for going much shorter than that would be more apparent with the Swede than with the smaller cased 6.5's. I don't have any real world data to back that up, though so keep in mind that I am taking a WAG on that particular aspect of the Swede.
Where twist is concerned, the consensus seems to be that an 8 twist will allow you to shoot anything up to the 140g VLD's with good performance. I have 4 6.5's ranging from my two Swedes up to a .264WM. The .264WM and both Swedes have 9 twist barrels. My 6.5-284 has an 8 twist.
The 6.5-284 has shot both 123g and 140g AMAX's very well under all conditions. Many people say that the 9 twist will not reliably stabilize the 140 class VLD's. My personal experience with my Swede has not borne this out. It delivers consistent accuracy with Berger 140g VLD's under varying atmospheric conditions down to an altitude of around 900 ft. ASL. The jury is still out on my other Swede and my .264 WM.
In short, an 8 twist should work with everything you would conceivably shoot out of a 6.5. The 9 twist may or may not work. It has worked for me so far, even with VLD's. However, I was prepared to go with something else if the VLD's didn't work out. If you have your heart set on shooting 140 VLD's, the 8 twist would be the safe choice.