It's pretty easy to have the MS control the radiator fans. You hook one side of the relay to +12 and the other side to the MS, which stays at +12 if the fan is to be off and gets pulled to ground if the fan is to be on, which will pull current through the relay turning it on.

For idle control, the stock ECU uses the coolant sensor, so if the stock ECU isn't able to get correct information about the tempature of the coolant the idle may be a little shaky.