It does not matter if one of the cables goes slack at full deflection, the surface will not flutter do to the aerodynamic forces places on the surface during deflection. The only cable doing any work is the one pulling. It is better to have the horn just behind the hinge than in front of it because if it is in front it will get tighter at full deflection and put more strain on your setup.

The pull pull horns and servo arms do not need to be the same size. It may be better to have a smaller servo arm for mechanical advantage.

Read the ackerman page. It is full of good info.