I asked PayPal a number of questions via their secure message service, and it was clear by the response they read none of them, so with your leave good people I will ask here

The email address I used when I originally signed up with PayPal, is the email address I put in my stock agency profiles for payment? Nothing else (other than ticking the PayPal option...)?

Also, I understand I could record another email address in PayPal, and use that one (my more photography related email address)? How do I do that?

Ta!

That's correct, you just use the email address you signed up with, but you can add a secondary address (or more if you want) in your account settings.