Is it OK to have a roof-mount power fan in conjunction with a whole-house fan?

It is unlikely a roof-mount or gable-mount power fan will provide sufficient exhaust airflow that a whole-house fan will require.

Whole House Fans, when running, overpower the attic ventilation system and turn every attic vent in the attic/on the roof into an "exhaust outlet" for the running whole-house fan. However, a roof-mount power fan will not be running 24/7 so it needs to be considered an opening on the roof (about 144 square inches of Net Free Area). As a result, it does not provide very much "airflow outlet" for the whole-house fan but it does provide some. There is a formula for properly sizing whole-house fans available on airvent.com.