Read
www.cavyspirit.com/sociallife.htm. Even bonded pairs can be successfully intro'd to other pigs and other pairs to live altogether. I've done it with a total of 7 sows.
Just make sure you do quarantine procedures before you introduce them. Quarantine is not just for new pigs coming in to your home but for your existing pigs as well. Quarantine is 3 weeks in a separate room.