Can I use a rheostat as a potentiometer?
Come on guys the answer is absolutly not! the way both reostats and potentiometers work is they absorb the heat not used on the load so you would be making a wall heater in your box,,, a modern dimmer uses SCR or TRIAC solid state devices that limit the amount of AC making it to the load. Dont use a standard Dimmer for floresent lamps unless the ballest are listed for this. most dimmable Floresent lamps require a special ballest and dimmer combination.
I thought I should specify, some old celing fans did use variable resistors to control the field winding (speed control) these can cause a fire if used as a dimmer, if the the device is a modern dimmer it would be ok to use it should have what it is made for listed on the body of the divice and how many watts it can handle.