The Wicked Device Motor Shield has 6 independent H-Bridge ICs on it, each capable of delivering up to 1.2A Average / 3A Peak Current, and with Motor Voltage up to 15VDC. It's suitability to a particular application depends on whether those parameters are satisfactory to the task. For stepper motors, the 3A rating is not really applicable, so go with the 1.2A as a limit.
Your motors will have properties of Amps per Phase (max) and Resistance per Phase. The "on resistance" of each H-Bridge is 0.3Ohms. The Motor shield also has 0.1Ohms series resistance per channel. So the total resistance on each channel is effectively R = (0.4Ohms + "Motor Resistance"). You should supply no more Voltage to the motors than "Motor Max Amps" * R, or you risk damaging your motor.
For 6-wire unipolar stepper motors the common wires (center taps) can be left floating and then what's left can be treated as a 4-wire bipolar stepper motor. Alternatively you can connect the the common wires together and treat the result as a 5-wire unipolar stepper motor. How you wire it and how you drive it are intimately related. Here is an example of how you could use the common [ref]. In a nutshell, if you use the common and drive one winding at a time, you can use less power (1/2 as much) at the expense of less torque (1/1.4 as much).
The WickedMotorShield library Wicked_Stepper class implicitly assumes a Bipolar drive strategy. You can tell that by the fact that it only takes two drive channels as an input. In order to implement a drive strategy for a unipolar motor, you need to use 4 drive channels.