I don't like to use a coincident pair on acoustic guitar because I get very different tones in each mic due to the different angles, and not necessarily in a good way. I used to record acoustic guitar that way when I started out, but felt that the blend of the two tones didn't sound as good as the tone of only one small diaphragm condenser placed "just so."
Now if I want a stereo acoustic guitar track I use a whole different setup with one small and one large diaphragm, but that's for another thread.
I use XY all the time (as Juber is there), but it depends on the song it's being thrown into. As gearitis said, it's not the widest perspective, but for many songs that's exactly what's needed. For those, XY is great.
For solo guitar, I've been using ORTF a lot lately, which was created to emulate how the human ears perceive live music. Two mics spaced about 10 inches apart, facing away from each other so that they create an arc of 110 degrees. It's the best of both worlds in that it's creates a wider perspective and remains very natural sounding.
The source would go right down the middle, between them.
How far away you record is up to you. You're looking to experiment - that's part of what you'll need to do. I typically start about a foot away and play around from there. If it's a noisy or reverberant room, you might not be able to get too far away or the mics will pick up the noise.
No rules about this one (except for the 110 degrees at ear's width rule, which is the whole point).