Your mentor is correct. When two aileron servos are connected either with a Y or hardwired together for one side of the aircraft such as a biplane then there is no need to reverse one servo as long as the mounting and pushrod orientation is the same in both the top and bottom wings.

On the high strength servo arms (metal) which are intended to minimise servo arm flexing it is my opinion that since you are not using very high torque servos to start with that I would be comfortable with the plastics. These are all choices on how you are going to fly the airplane and only you can really answer that.

John