Would need a timer to do it.That is definetly a positive.
I wired from G on the furnace to G on the thermostat. Now I can turn the fan on or off - to act like a summer fan. I cut in 2 cold air returns in the basement, one on either end of the return.
Would that cause any problems I'm unaware of?
Depends how cold the air is coming into the furnace.
Also I am not sure about the thermostat, I have heard that this may be a multistage fan and that I should get a thermostat that can run the fan at different speeds?
Multispeed fans aren't controlled by the thermostat as far as what speed they use.
And thirdly, is there any way to have the "summer fan" I wired in - have it kick in at a certain tempurature or certain time of day...then I wouldn't have to wake up and manually turn the summer fan on.
I'm not sure what pipe you mean? Is this the pipe that comes from outside?The humid air in the combustion intake pipe is what will condensate in the furnace heat exchanger.