New guinea pig introduction?

Question
Hello!

I have had my guinea pig, Petunia for about five years now.  She has an extremely tame personality and has never created any conflict with other guinea pigs.  Her cage mate passed away a while ago, and she got lonely. I was going to go out on the search for a new guinea pig friend this week, but yesterday when I was at a pet store, there was a guinea pig that was for adoption for free.  Apparently she was a mother at one point and she is 2 years old now.  She was friendly with other guinea pigs and the previous owners were moving so they couldn't keep her. I decided that it was the perfect opportunity to adopt her and bring her home to Petunia.  

 When I did, I put her in a separate cage next to Petunias.  After a while, I decided to introduce them to each other on neutral ground.  I set a towel on the couch and sat with them.  At first they were fine, and didn't really associate with each other closely because I gave them pieces of celery.  Afterward, Petunia tried to snuggle with her (Petunia loves to snuggle and is really sweet to other guinea pigs# but the new guinea pig #Harriet) freaked out.  She squealed and jumped.  After that, they sniffed each other a bit but then Harriet nipped Petunia on the nose.  Not hard at all, and Petunia just sat there patiently.  Today I put them together again on the couch and Harriet did the same thing.  She is fine if Petunia doesn't try to snuggle with her, but I feel bad because I wanted to get a guinea pig that would feel mutual about it. It also worried me a little bit that Harriet is nipping at Petunia, and sometimes she chatters her teeth.  I knew going into that it maybe woudn't be an easy introduction because Harriet is already a grown adult and has babies she most likely bossed around.  I know introducing a baby to Petunia would be better, but I thought adopting an older one was better because it was older and probably woudn't have gotten adopted as soon as the younger ones.  Any advice you can give me?

Thanks!

Answer
The new pig is just reacting to a new surrounding. Very often it's the resident pig that starts that kind of introduction, but for some reason Harriet is making the gestures first. I think it's probably because she is frightened, not aggressive.

Unless they are going into a rolling, tumbling, fur flying blood drawing battle I would just leave them together. They will work it out themselves. Minor pushing and shoving is normal. Since Petunia is so submissive it shouldn't take long before they warm up to each other.

It's also possible that Harriet has not had any other roommates except for an occasional boar, so she has always been the "queen of the house. It will take her a little time to learn how to share space with another sow, but she will indeed learn.

It typically takes about three days before they finally give up the battle and just settle in. Be patient and I think they will be fine.

If of course they are drawing blood on one another that's a different story, but in most cases it's just a matter of adjustment and acclimation to the new household.