Can the furnace fan speed be controlled by a rheostat or other device?

Yes but it is not recommended. Mimimum air requirements,

Can I ask why? There is a device for motors that will cause the blower to gradually increase and decrease RPM, but they shouldn't be used on cheaply made motors.
IAQ Direct - About FanHandler Inc.

Can I ask why? You sure can.
BTW A friend of mine owns fanhandler and his name is richard and there products are great for variable speed TEMPERATURE controlled systems.
The person who asked the question mentioned a rheostat and that is a speed controler that does not take into consideration the temperatures in the system. Burning up the heat exchanger or freezing up the evaporator by the use of a manual control is not my idea of a smart move. The Fanhandler is a seemless speed controler that will speed up automatically when necessary to keep the system from a overload condition. A rheostat will not do this and in my opinion is unsafe. Also a capacitor run motor does not like a rheostat anyway since a rheostat drops voltage and blower motors do not like that without some sort of diode/rectifier circuit. Been there done that got the shirt got the hat. I also thought I would mention that richard is not what the Fanhandler guy goes by but this site will not allow me to use his regular name. I will spell it backwards for you (kcid).

Can I ask why was not meant for you hvac1000. I think we both know why airflow adjustments should only be done by qualified service tech's. I also agree, don't use a rheostat, that is why I told them of fan handler.