How can I get Google Photos app to see old/other photos stored on SD card?
First check if in Google photos app options under Device Folders if you can see the photos on external SD. 
If you can't,  from Settings &rarr; Apps &rarr; show System apps &rarr; Media Storage &rarr; Clear data (this is the path in Marshmallow ). Don't worry, you won't lose anything
Reboot and wait for a while for media index to be rebuilt


You should now see photos on external SD