Most of the pigs in rescues originally came from pet stores, too, and they won’t always know their exact ages. You’d have the benefit of having them properly sexed and health checked (most of the time) through a rescue, but they aren’t magical places with pedigreed pigs. They just happen to be the second-to-last stop for all the unwanted pet store and Craigslist pigs that inevitably end up at a shelter anyway.
Have you researched how to sex pigs? I’m not 100% confident in my ability so I took my two “female” CL pigs with the understanding that if they turned out to be male, I could either return them or find a suitable home myself. Thankfully, I got it right but neither of their previous owners wanted them back or seemed to care how I might have re-homed them.
I understand that you might be hesitant to take in an unknown pig if you can only have one more and need it to be compatible with the one you already have. But, there are no guarantees and Petco certainly isn’t the way to go about having a pleasant experience. Educate yourself, find a good vet and contact a
GP rescue. Even if it doesn’t work out for you to get a
GP from them, they will help you make the best choices for both you and the pig you already have.