The twist rates are inherent to the barrel, and is not dependent on length.
Sorry, I wanted to expand, but hit the "Submit" key too early.
The twist rate is inherent to the barrel, and is an option that you should select/keep in mind while shopping just as you would the barrel material length, profile, external finish, muzzle type, bore plating, etc., as well as on the AR the gas system length.
The overall consensus is that the faster rate barrels (smaller denominator) should favor a heavier/longer bullet, while slower (larger denominator) favor a lighter bullet.
Currently, most think of a 62 gr. .223/5.56 projectile as the diving line between faster and slower twist-rate barrels, with the 1:8 or even 1:9 being astride this divide - any heavier/longer, and you should go with a 1:7, any lighter, then a 1:12.
In truth, this has a lot more to do with the individual barrel selected, as well as the individual ammo used, and is also heavily dependent on the end-user's need for any particular levels of accuracy.
For example, at most high-end "tactical/defensive" AR classes, you'll see a lot of "mil-spec" carbines with 14.5 to 16-inch 1:7 barrels. With round-counts in the 300 to 1K per day range, as you can imagine, as-cheap-as-you-can-get range-fodder quality ammo is preferred by most students, and 55 gr. ammo is typically favored, particularly for shooting steel targets (as, still, to this day, there is a residual belief by many that the 62 gr. M855 is somehow deleterious to quality AR500 targets that are designed for the purpose).
Zeroing is typically performed at the 50 or 100 yard line, and not only are keyholes rarely witnessed. Similarly, shooters of these 1:7 twist-rate guns often reach out as far as 300 yards using unmagnified optics or ironsights. For example, quarter-scale IPSC plates were used at the Alliance PD facility last weekend, for the "Friends of Pat" memorial celebration and training gathering, even beginner-level shooters were readily and consistently engaging at the 300. Personally, as a true beginner with long-guns, with the 16" 1:7 barrel on my DDM4V5LW, I was easily tossing shot after shot after shot onto an IPSC A-zone, from the prone, at 150 yards, using steel-case Wolf 55 gr. ammo, and I was having no trouble at all shooting 8" movers at the 100, in a dynamic manner.
But that's a rather forgiving way to interpret the BSA template.
Unfortunately I do not have the personal experience or knowledge required to really comment on varmint shooting needs in terms of accuracy. Here, not only a more exacting match-up between barrel twist rate and bullet weight may be required, but the needs of this activity may also carry over with implications for the barrel profile, muzzle device/crown selection, as well as other issues such as load selection.