Should I get another guinea pig?

I have two guinea pigs now, Frances and Celia. I'm thinking about getting another one, should I? The two I have right now are 1 and 1 1/2 years old, and I will be getting a baby. I have a big enough cage. My parents said I can get another one, but the people at petco and petsmart told them that they would all fight. So should I get another one???