I missed the pot.
As far as I know, the fan is going to look pretty ohmic, so you're going to be limiting current by drooping the voltage, right? Bear in mind that that will reduce the torque- fast and by a lot. And you're right, if the motor stalls, your current will be high- possibly high enough to damage the motor.
A better fit for speed control would be some form of PWM. That way, your current stays high, so your torque doesn't drop off, but the torque is being applied only sporadically.
If you want, I can give you a PWM controller IC and a power FET that'll get you where you are trying to be- a knob which will control the speed of your stirring motor.