Do you have the space and funds for two pairs if it doesn't work out? Ideally you would quarantine this piggy for two weeks away from your existing pair and then do a bonding session on neutral ground. Sows can be awkward when they decide they aren't accepting another so it's impossible to know what will happen or if one will get left out etc. If you have the space to expand to a group of 4 or to split into 2 pairs then you at least know that you have a back up plan. I would recommend not looking on Craigslist in future as there will always be these piggies that need "rescuing" and we can't save them all unfortunately