Hmmmmm....Tuning Servos.. There have been entire books written on this subject. From a user perspective it really means getting the system, in this case your milling machine, to operate smoothly and accurately. This is normally as simple as making a few controller adjustments. It has to be done with with all drive system types.
Lets use the car analogy. You are the controller, the car is the motor, and the feedback is all of the information that you are processing (sight, sound, seat of the pants feel), and you output signals to the throttle, brakes, and steering. This is a closed loop system. For simplicity we will ignore everything except the throttle and the brakes for this explanation.
Let's say you are sitting at a stop sign and a block down the street there is another stop sign. So the ideal motion profile is to accelerate smoothly and quickly up to the target speed (speed limit), proceed down the street at the target speed, then decelerate smoothly and quickly to reach the target position (the stop sign). A new driver might accelerate too quickly or too slowly, not be able to control the speed, and brake too hard or not enough. So in the case of the servo system, you have to teach the controller how to operate the car. This is tuning.
Many modern servo drives can be operated manually with very simple controls. A FOR/REV switch, an ON/OFF switch, and a speed pot much like a VFD. The setup is easy by making adjustments to the on-board software via the front key pad or connecting it to a computer. The parameters are normally Acceleration, Deceleration, Max Speed, and the Proportional Integral Derivative parameters (PID). I'm not going to try to explain the PID here, but normally the only value that you have to adjust the P parameter. This sets the ''tightness'' of the system, set to high and the system becomes unstable and the motor will oscillate, set too low the system is mushy. For manual operation not so critical, but for CNC operation it has to be right. Not difficult to do, just takes a bit of playing around to see how the machine reacts to the changes you make.
Speed range for rapids..... I have my mill set to 100 IPM rapid. Plenty fast. Slow jog is set to 30 IPM. Some modern CNC machines run 1000 IPM rapids, that's just crazy and a lot of horsepower on the servos.
I also want to note here that the reason I suggested 1200 oz-in steppers in my post above is that you are not using ballscrews in the current setup. An Acme leadscrew requires about 8 times the torque of a ballscrew under the same load conditions. Should you decide to put in ballscrews later, you can always turn the torque down on the motor. A simple adjustment. It's better to have it and not use it, than to need it and not have it. You can also adjust the torque on servo motors, again a simple adjustment. My Z axis stepper for instance is turned down to 50% torque output, which gives me about 350 lbs of max down force on my quill before the motor decouples. My shear screws will shear at about 375 lbs in case something goes horribly wrong.
A reduction is a good idea.
Understanding a PID loop is half the battle.