Can I get just one rectangular tile to use in a textile CAD program or a web browser (as a web page background)?
Yes, SymmetryShop saves the exact tile with your artwork as a channel. You can open your file, apply the SymmetryShop Tile selection, crop, and get the exact rectangle that seamlessly tiles the plane. You can also create a Photoshop pattern preset from the SymmetryShop Tile and use that as a pattern fill.