I See FF is a little curious about this, me too.
Bob - Those 4" Pers. Computer type fans draw ridiculously low amounts of current/power (about 10-20watts). It is not logical that the fan's current draw would affect your light output for a few reasons - If both the lights and the fan were plugged into the same outet and that outlet could not provide you with the current you required to continuously run those devices other bigger, badder things would happen, like fuses blowing and wires burin' up. Keep in mind that household outlets typically run between 10-20Amps at 120Volts thats 1200 Watts to 2400 watts. Doubt your 20 watt fan has much impact there. So I think you can plug the fan in anywhere.
Also, Compacts are basically flour. lights and these are not voltage/current dimable, i.e. when the voltage or available current drops below the required level for the balast to keep the ionization of the tube gases from firing then you would likely see other things happen like bulb flicker rather than a consistent dimness. Not 100% sure about this but pretty sure.
I'm confused about this one. Is it possible that you looked at the light with the room lights on differently - for example before installing the fans the room lights were off and the compacts looked brighter, after you installed the fan you looked at the lights with the room lights on (hence the compacts would look dimmer)?
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein (1879-1955)