Guinea pig helpp???? please answer?

ok well i have a guinea pig named Pebbles. She is big and a little chunky. We are really not sure how old she is. Maybe 6 or 7 months. Anyway, i want to get another guinea pig and my mom is thinking about it and my stepdad says absolutely not because they will fight. I told him that we can take her back if they fight the very next day. My guinea pig Pebbles is a girl and we are thinking about getting another girl. We don't want to get another cage because it is double the cleaning of the cages, food, water, all that stuff. I told my mom i will love both of them and i promised. We looked at the guinea pig we kinda want yesturday. She is adorable and she is black and white and calm and doesn't wiggle as much as the other ones at the pet store. How can i convince my mom to let me get another guinea pig, ALSO is it a good idea to put to guinea pigs together? females? we are just afraid they are going to fight. but my guinea pig Pebbles is friendly and only really NIBBLES, not bights.