When you create new overlays, they'll automatically appear at the far right of their Layer. But free will exists, even in the world of Spark!
To re-arrange overlays within a Layer, simply click and hold briefly on an overlay before dragging it to the right or left. That short pause after clicking indicates to Spark that you aren't trying to edit or take an overlay to air.
You can change the order of overlays in either Edit Mode or On-Air Mode, as well as while sequences are running.