Hello,  You can control the speed of a universal motor such as a vacuum
cleaner motor with a standard incandescent lamp dimmer.  You can also
use a high wattage resistor of the right value but it will generate
a lot of heat.  You might even be able to find a light bulb that will
reduce the speed enough.  Another possibility is a small variable
autotransformer, commonly known as a Variac.

You can have as many voltage controlling devices as you want and
select between them with a switch or relay.  Make sure that the switch
will not connect two devices at the same time or you may have problems.
Not a problem if you use a resistor but it might kill a dimmer.

A fairly simple mechanical solution would be to make a "flow divider"
that puts a small hole in the supply to the wind motor and a reasonably
big hole to the atmosphere.  If the big hole is big enough you don't
even need the little one but the regulation will be better with the two
hole solution.  A centrifugal pump will move a lot of air at low load
conditions.

Best regards,
Spencer Chase