Also, 18 is a pretty average number, I'd say. Poppins poops about 3x the amount Buster does, and his poops are bigger. I have always attributed it to activity level (more running=more eating=more pooping) and possibly age (because everyone gets stuff stuck to the walls of their intestines as the age and it can build up slightly, making feces smaller, if that makes any sense)
Guinea pigs are different from other rodents (like rats or mice) in that they have sphincters which they are in control of...to an extent. Mice and rats just poop and pee as the walk because they have no control whatsoever (no sphincters). I don't believe guinea pigs can be potty-trained if they don't want to be. I feel mine do a pretty good job by letting me know when they have to go during lap time but it's not something I "taught" them.
Side note: most rabbits will automatically use a litterbox if it is available. My bunnies were always "free range" and used litterboxes just like the cats. I tell anyone I know that has a bunny to try this, and I haven't had anyone complain about it not working yet. Most of them are amazed. :)
At first I got my guinea pigs thinking they would be like rabbits in that way but they are NOT! lol