Hi gabe,
From the data sheet I guess the motor is a permanent magnet type with wound rotor.
For fan use, the output flow is proportional to speed, and the power consumption is proportional to speed cubed.
The other thing about the DC motor is that as the speed is varied, the rotor current is essentially constant at constant torque. So, depending on the application, a DC motor running at a variable speed MAY need to have an external cooling fan. You will see this with proper DC motors made for industrial applications having external cooling fans.
For some applications however, the motor cooling is obtained by a simple fan attached to the rotor shaft. These motors are mostly unsuitable for variable speed uses unless you can assure that the motor temperature rise will be within the maximum value.
The motor cooling air flow will be less at low speeds but so should the temperature rise because of the changing (reducing) load at lower speeds. To get an air flow of one tenth of the rated air flow, the rotor speed would be about one third of the full load speed. The data sheet shows that as the back pressure is increased, the flow drops (usual) but the flow is 60 CFM at a back pressure of 1 inch. So it appears that the fan motor will be OK at a flow rate of one tenth the maximum flow rate.
My approach would be to set up the fan on a bench without baffles etc, and check the motor current as the speed is varied and calculate the power dissipation over the speed range. If the dissipation reduces as the speed reduces, then you can be confident the motor will not overheat. Alternatively, a temperature probe to measure the actual temperature rise would be good.
hope this helps.