Fog lights aren't supposed to be on with the high beams simply because if you were driving in heavy fog, when you actually needed the fog lights, the high beam would simply reflect off the fog and blind you, and you wouldn't see jack. Fog lights illuminate low to the ground for maximum visibility in thick fog.......the charging system will handle all the lights fine.
Anyway, if you tap into a "hot" wire, something that is on with the car being on, you are essentially adding a parallel leg to the circuit you tapped into. This will pull more amperage through that circuit, and if you expect to run the lights off it, that is a BIG boost in amperage. You run the possibility of frying a control module by doing that, or melting wires...
I wouldn't bother.