It sounds like an ambitious but interesting project. I have not had any experience with anything similar, but I can at least make a suggestion here:
but i do not yet know how I can measure the rpm yet on this spinning mirror.
You could have a separate laser/sensor pair mounted perpendicular to the axis of the rotating mirror. The sensor would be covered with a slot so that only when a face of the mirror is at exactly 90 degrees to the sensor would it detect the light from the laser. That is, the sensor would be triggered 6 times during the rotation of the hexagonal mirror. The rotational speed of the mirror could then be calculated from the interval between sensor pulses.
1000 pulses per second from the sensor would indicate a rotational speed of 10,000 RPM (assuming a 6 face mirror).