The spec is 9V minimum but 8 volt could be just about sufficient to turn on the T100MX+ optoisolated inputs. In the worst case scenario you can solder in parallel a resistor to the current limiting resistor onboard the PLC to increase the sensitivities.
500 Hz PWM signal is not a problem. You can use the PULSEWIDTH and PULSEPERIOD function to accurately measure the duty cycle of the PWM inputs.