If I can have 10 pets with a GA, what happens to them when my Gold Acct expires - will I be able to keep all 10 of them or does it go back down to 5 pets only? Who decides what pets stay and which ones go?

You get to keep them afterwards, but once you move a pet the extra slot is gone. When your gold account expires you're going to have 10/5 slots. If you move a pet, you'll have 9/5 slots.

You can't adopt a new pet then unless you only use 4/5 slots.