In a tractor config, the prop wash travels over the rudder ensuring some airflow. However, it will be more of a challenge to balance out your model correctly if the motor is in front. In my opinion, it isn't worth the effort to place the motor in the nose. At high AoA, aileron and elevator are more critical. I really think your design is fine as you've laid it out. I'll dig in my books for rudder area proportions, but the 15%-20% that Jim mentioned sounds about right.