Some run the Reactor post return pump, some run the reactor on a separate loop inside the sump area, but this requires another pump/powerhead(albeit a small one).
The sump area near the return pump then takes that CO2 rich water back to the main tank. This latter method seems to do best for removing the mist.
And with a smaller rate of flow, the mist is still produced, but you have more dwell time and less flow in the large reactors.
Larger gas bubbles burped out rise in the sump instead of the tank also.
There's a balance between too much flow through the reactor, (return pumps often have issues there), versus too little obviously(like ario 180 on a 200 gallon tank etc).
If you have a larger powerhead for the in sump design, then use a flow control valve, this is the best option IME.
Do you need a special needle wheel impeller?
No. Fraying the impeller dIY should be fine I think.