Although it seems that slow twist barrels are usually longer and medium twist barrels are usually shorter, the rate of twist is quite independent of barrel length.
A couple of examples are the Cabela's/Pedersoli Blue Ridge rifles which have 39 inch barrels and a 1 in 48" twist, and the Traditions Crockett rifle which is a 1 in 48" 32 caliber with a 32 inch barrel.

http://tinyurl.com/mmtvto


There have been a few short slow twist gun barrels in the past but not very many.
Accuracy is based on stabilizing a projectile by spinning it with the correct amount of velocity creating revolutions per minute (RPM's). The optimum amount of RPM's required is based on the length of the projectile and the Greenhill formula. The longer the projectile, the more spin that's needed to stabilize it.
Balls are shorter and require less spin, depending on caliber.
Conical bullets are longer and require more spin, how much depends on their length (which is also related to their weight).

Adding more velocity increases RPM's until the sweet spot which produces better accuracy is found.

To reduce RPM's, load less powder.
To increase RPM's add more powder.

So accuracy depends on the length of the projectile being fired and how much velocity and RPM's is imparted to it. Conicals usually do require a faster twist unless it's a very short concial like a Buffalo Ballet which can be driven at high velocity from a slow twist barrel like the Great Plains.
If one wants to shoot a patched round ball at hunting velocity, then a slow twist is usually better but not always. It also depends on the quality of the rifling itself.
If one wants to shoot a longer conical, then a relatively faster twist producing more RPM's is required.

To reiterate, fine tuning RPM's is controlled by the amount of powder.
The choice of twist rate should be influenced by which projectile is intended to be fired for the primary purpose of the rifle.