I just did one using 8 1/2" starting squares and 5/8" seam allowance.
If you don't sew the x across then all 3 layers will have a chance to bunch up. Not sure what the smaller size would feel like. I can recommend using nothing smaller than a 1/2" seam allowance because you need to trim each seam and that is difficult when the seam gets too small. When sewing the rows together, make sure to open the seams so you have the seam bulk distributed. Don't forget to sew one seam all around the outer edge. Not sure what you mean about the corners. I just sewed edge to edge in all directions.
BTW: I didn't want all that fuzz in the washer so I used a chenille brush to brush the seams first. I spritzed them with a little water and just kept brushing. Unbelievable how much thread came off.
Here is what the final result looks like.