in a very general way, the UNO/ NANO / Pro Mini are the same processor with almost the same number of pins.
google Arduino RPM and you will find a lot of ways to read it.
a simple IR device that sends out a signal, then bounces off a freflective ( reflective to IR that is ) and then receives that signal. often some white paint on the metal flywheel or gear.
a magnetic sensor that detects the presence of a magnet on the flywheel. treadmills for instance
a gear tooth sensor......
a beam break sensor. take apart an old ball style mouse and see the little wheels with spindles. one emitter LED and one reciver, the spindle breaks the beam.
all the above send a pulse. all are seen the exact same way to the Arduino. some have one pulse per revolution, others many pulses, but every one has a duration ( time) of a revolution.
your need to have an event at some speed is a beginner application and should be satisfying for you to achieve.
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#2, I a not sure what a stepper motor for a servo valve is.
but I am pretty confident you can control a motor. the only requirement is that you understand the terms.
controller = logic device, ie : microcontroller works on tiny power signals.
driver = power regulation device that takes large power and regulates it to the motor, gets is signals from the controller.
the controller , ie: Arduino cannot handle power, too many LED's and you burn it out.
the DRIVER can handle motors as large as any human has ever make. 10,000 horsepower....
what the Arduino does is control the DRIVER.
the driver sees the signals and drives the motor.
you might make your own driver with some transistors or MOSFETS, or buy one that matches your motor. either way, yes, you can control it based on the RPM you measured in step #1.