The icons are ordered by marker group type first (in the same order as the display in the Map Marker task pane), and then by ascending numerical value of the icons. (Each stock icon has an internal number). Even if you create extra marker groups and fill them with stock icons in a different order, they are still displayed in their numerical order on the topics. If you use custom markers, they display in the order in which they were assigned to the topic. So it seems that there is no solution for managing this.