When 2+ tool items are added with plugin_add_toolbar_item(), they are ordered like this: 2 3 4 5 | 1 Quit. The internal plugin_items list order is correct, so rebuilding the toolbar (for example by Edit -> Preferences and adding/removing an item) places them in the right order: | 1 2 3 4 5 Quit, as described in function /**.
Looking at plugin_add_toolbar_item() source, it's obvious that the second and subsequent items are always added before the current separator position. Adding them after | would require counting; it's easier and shorter to always use toolbar_get_insert_position().
Geany 2012-30-04 / Debian wheezy/sid (doesn't matter).