What's the best way to make sure users of product 1 see only articles related to product 1? At first glance, I thought an article rule using Referral URL. Thoughts?

Without knowing how you are set up - this may or may not be useful. You could tag all "product 1" users and then use an article rule requiring the user to have that tag for visibility. This works best if you have a way to automate the tagging of your users - or it won't scale if you manually have to tag every person that buys your products.