If you are using the monitor out of the focusrite, turn down the knob on the unit or in the software or turn down the volume on the monitors...
I always use 2 seperate tracks because it's nearly impossible to get the preamps to have the exact output and volume can change as the player move, it's easyer to change volumes on seperate tracks and have independant compressors.

PC has given you tips on the monitoring settings.

For microphone positioning, the basic question you have to answer is whether you are going to set up the microphones as a stereo pair (e.g. in X-Y configuration), or whether you will use them as two mono microphones that you will combine later at mixdown.

Setting them up as an X-Y stereo pair reduces phasing problems and gets you a guaranteed stereo image, but is quite sensitive to the performer moving around during playing. You also capture the sonic image of the guitar at the specific distance of where you set the mics and have little room changing the sonics at mixdown, unless you are fond of added reverb or you also record a another room mic positioned considerably further back from the instrument.

Setting up as two independent mono mics gives you the option of capturing a close sound with one mic and a further sound with the other, which might look as though it gives you the flexibility of adjusting the acoustics in the mix, but in practice can easily land you with a less than perfect stereo image and phase problems between the two mics.

My recommendation is that you start with the mics in an X-Y stereo configuration and set Garageband up to record a stereo track. A good starting position for the microphones is at right angles to the twelfth fret and about 12-15 inches from the instrument, pointing towards the sound hole. If your performer is willing, you should record him (her?) playing the same piece lots of times but with different positioning of the X-Y pair of mics relative to the guitar. For each position, make a careful note of exactly where the mic is (XYZ co-ordinates relative to the performer's feet and also the direction of where the centreline of the mic is pointing), then take your time to listen to the replays and both of you decide which positioning best conveys the sound that the performer wants to put over to a listener. Having the mic a little lower than the instrument and aimed up slightly can reduce the problem of reflections off the floor, but may bring in unwanted effects from other surfaces in the room.

Whatever you decide to go with will not necessarily be a universal best position, as the floor covering and other room acoustics play a part, not to mention the style of what is being played and whether this is a solo track or has to fit in underneath a vocal or with other instruments.