Got to agree with Bruce, if somebody told you to connect the rudder and aileron servos then you got a batch of bum info. Aileron and rudder need to be on separate channels.

As far as the plane itself. I am an instructor at our field and I currently have a student that came out with the Nexstar. You've done a good thing disconnecting the flight stabilizer and removing the speed brakes, go ahead and finish the job by removing the airfoil extensions. Once all that is done the nexstar flies like just about any other trainer on the market today. Make sure you assembly is good and all flight controls are securely fastened, but your instructor should check all of that for you. You should be fine.

Hope this helps