Is there a different shortcode for 2.x?
Yes, but not only is the shortcode different, the new version of the plugin (v2) is completely re-written from scratch and is therefore not backward-compatible.
Please see here for details.
I recall tweaking the gallery to give me rectangular thumbnails, different spacing in the thumbnails,
Bear in mind that WP-SimpleViewer v1.5.4 uses SimpleViewer v1.9 rather than SimpleViewer v2.1.3.
SimpleViewer Standard settings are determined by the options in the gallery's XML file ('gallery.xml') file in the gallery folder (in the 'wp-content/photos/' directory). These options can be changed by managing the gallery via the 'WP-SV Galleries' link in the Media section of the WordPress Dashboard.
In order to incorporate Pro options (such as those required to change the thumbnail size), please see here: http://simpleviewer.net/simpleviewer/pr … tions.html
and removing the "SimpleViewer" banner link
This can be achieved by upgrading WP-SimpleViewer from SimpleViewer Standard to SimpleViewer Pro by the replacing the 'viewer.swf' file in the WP-SimpleViewer plugin with the SWF file from the SimpleViewer Pro v1.9 download package.
SimpleViewer Support Team