Introducing a new piggy

So about a week ago, I got an 8week old female guinea pig. I am trying to introduce her to my other female guinea pig who is about 6months old. The older female is nice to her at first, then starts chasing her and nipping her in the behind.Then, the younger piggy squeaks very loud but does not have any injuries. Does anyone have any suggestions to make things go smoother? Has anyone had this problem before? The older guinea pig is friends with my rabbit so I thought she would be happy to have a new piggy around. Please help :)