Hello there,
This workflow may work for you.
1) Add the 500GB catalog to ArcMap table of contents
2) Make a selection on the 3500 tiles through the raster catalog attribute table.
3) Right click on the raster catalog in the table of contents and choose Data -> Export Footprint.
4) Then run the Copy Feature GP tool, copy the footprint to a feature class.
Then you can use this polygon feature class to merge with other polygon layer.
Jie