Male guinea pigs tend to fight unless they are introduced at a young age, so be careful there. Females tend to accept each other easier.
Your piggy is being normal by hiding - the way to break that is to - get the pig to trust. That's easy - take them out, be sure to support his little butt. Put him on your lap, and pet him, and talk softly. Do this every day, and add more time each day. Give him floor time, sit on the floor, and let him run around and play. Call him, and if he comes give him a treat.
Do you give your piggy a daily salad? Try hand feeding him instead of in a salad bowl.
Pigs are the sweetest gentlest little creatures, and they thrive on attention and affection.
Hope this helps.
Dallas, Houston & Peanut the Prairie Dogs
Gypsy & Amber the Satos
Apache, Amigo, Max, and Morgan the Kitties
Scooter, Annie, Buttercup & Squeaky the Guinea Pigs
Mocha & Latte the Rat A Tats