Should you put cross stitch behind glass?
I don’t recommend leaving your cross stitched projects sandwiched between glass. This compresses/flattens the stitches which can make them look odd.
What kind of thread do you use for canvas?
Thread. Heavy-duty thread (size 40) made from polyester, cotton-wrapped polyester or cotton are well suited to a heavier-weight canvas. Wool and upholstery thread are also very strong threads often used when sewing with canvas.