I don't see a problem with using a stepper for your Z.
The advantages of a servo are closed loop positioning. With a properly sized stepper, you shouldn't loose steps anyway.
The advantages of the stepper on the Z axis are the higher holding torques at lower RPM to support your Z weight that isn't counterbalanced. You could use a servo with gear reduction but it can be a more complicated design.
The advantages of a servo over a closed loop stepper is the rated torque remains constant (sometimes it drops off after 3000 RPM or so, it depends on the servo, it's a general statement) throughout the RPM range of the servo, while the torque from a stepper drops off quickly as RPM increases.
So if you're not trying to match the speed capabilities of your Z with your X and Y, then I don't see a problem with what you propose to do.