I too would add sashing because it will be a lot easier to sew the blocks to sashing than to each other. But I WOULD add cornerstones--even if they are the same fabric as the sashing. They will help keep the blocks lined up properly. I don't think it looks tremendously matchy-matchy, but I think a few randomly placed bright blocks would completely change the look of it. As far as the width of the sashing, the blocks are small so the sashing should be narrow.
Another suggestion would be to surround each block with a border and then sash. This idea might be helpful if you need to get all the blocks to a uniform size and don't want to trim them all to the smallest common denominator.