RE: how light is too light.
Since arrow speed is the least important aspect of bowhunting, I pay no attention to it. The things that are imporant - 1) A FOC that is high enough to get good arrow flight from my broadheads, when shooting in wind. 2) An arrow spined properly for my setup. 3)An arrow that has been spine-tuned to shoot it's best.
If a hunter approaches his arrow selection in this manner, it will be literally impossible to shoot a super light arrow. If he approaches it by looking for speed, he'll almost always end up with a setup that doesn't give broadhead flight as good as it could be. When I get done setting up an arrow for my bow, the speed is what it is. It's always fast enough. I have no speed goal in mind when starting, and would never make a decision based on whether it made the arrow faster.