I have a 7" Octane stab on my Athens Ibex with a 1 oz weight on the end. This setup works for me because it creates the forward/backward balance I'm looking for at full draw. The result is better aiming. The stab does provide some vibration reduction on the shot, but that's not why I use it. That's just a bonus.
The reason why the former Olympian says hunt with the longest under 12" is because the longer the stab the less weight on the end of it you need to balance the bow. Less weight is less weight you have to hold at full draw and in a hunting situation you may have to hold for much longer than normal to get that good shot. Also, for hunting anything more than 12" can generally be an issue with getting hung up on bushes, trees, etc.
Read the PDF I attached. It'll tell you everything you ever wanted to know about bow stabilization. Also, ask yourself what am I trying to achieve by using a stab --- bow balance, vibration reduction, or both (with one possibly being a priority over the other).