It doesn't really matter what servo arm you use. On things like the elevator and rudder and ailerons, you are better to put it farther out on the arm, but it is not necessary. Just make sure you have enough movement. The throttle arm leangth doesn't matter as much either, just make sure you have full control to open it and close it. That usually takes a longer arm.

If you put them farther out on the servo arm, it makes for a more positive control on the surface generally. I tend to move the servos so I can use the 5 star arms and cut off the excess arms too so they don't hit.

Is this your first plane? If so, I hope you have someone to help fly it. It isn't a trainer and will be pretty fast. It is a good plane though. If not your first, you will have fun with it.