The smoke resistor would be running on the 5-7 volts DC from the AC-DC power supply. The fan motor wants 5VDC maximum, and less if you don't want the smoke coming out really fast. Given that, I'd use a resistor from 20-100 ohms in series with the fan to get the right fan speed for the effect desired. The resistor would probably want to be a 1/2 watt or larger.
My advice for smoke units is the bigger the better as far as reservoir size, less maintenance. Remember, you also have to install it where you can access it for maintenance.
