Selection tool adds 1 pixel around my selection, even with snap to guides.

Press Ctrl + R then look at Ruler. Check that guide is properly positioned at pixel (marching ants on screenshot) not between two pixels (guide on screenshot).

Please zoom way in. Check to see if your guides are at the edges of actual pixel squares, or situated in the middle of them.If you randomly create guides without watching where they end up (at the edge of pixels), you'll be disappointed because Photoshop only makes selections based on pixel edges. So, even with snapping to guides turned on, you'll get whole pixels.

One workaround may be to double your pixel resolution (or any variation of making a denser image). That way, your pixels seem smaller, relative to the content you've created, so you can create your guides at smaller increments.