With the case of stepper motors, what correlates to the max RPM of the individual unit?
This particular datasheet doesn't provide much information about the power curve of the motor it describes.  A better example is this (not endorsing the particular motor, just picked at random for the information it presents).   The important bit is the graph at the bottom of the second page, which shows, for each of the motors the datasheet describes, a curve relating frequency to torque, at a particular voltage-current.  

Reading that graph for the detail you need to know is basically straitforward.  You will need to know the torque you expect to need for your application, and find the corresponding frequency.  Each oscillation of input represents four full steps of motion, and since these motors are 1.8 deg/step, you can take that frequency and divide by 50 to get the RPM.  

For example, supposing you needed about 0.2 Newton meters of torque, and went with the biggest motor on that datasheet, you would find that it could operate at about 2500 Hz, which equates to about 50 RPM.  However, it's usually wise to derate stepper motors in an open loop configuration to minimize the likelyhood of missing steps, so it might be best to run that motor at around 30 RPM so that you can have around 0.30 Nm at hand.