I would really read the link I posted above... Well I got the memo that you read it after I posted this.. Oops! Guinea pig introductions look REALLY scary to humans, but they're necessary for piggies, and can include lunging and nipping (and showing teeth, chasing, and lots of other scary things) so you need a big indoor area like a big bathroom or the kitchen, or a large pen in another room. As long as no blood is drawn then you just keep going. It can take several hours sometimes but eventually they figure it out. If your cage is less than 7.5 sq ft then don't bother introducing them until it's bigger though.
I had a new piggie, Clarissa, who was introduced to my herd of 4 sows on Friday. She and Bridget had a giant tiff and were flying at each other and fighting with teeth, so I separated, checked for wounds, and seeing none I put them both back in. Clarissa and Penny did the same thing. Clarissa chased Mirabelle, and Lacey was cowering in a corner for a while. It's normal. Sniffing butts is also a part of the introduction process, and no matter how much you tell Piper to not do it, she will do it until she and Minnie can sort out the all important question of who is boss and then get on with their lives. As long as Piper isn't bleeding, you've got to keep her in the neutral area with Minnie. Repeatedly putting them together and taking them apart is very stressful for the piggies.
Also I think Piper's bottom is doomed to stay yellow, I had a black and white Teddy (which is the coat type I think Piper has) and her behind was always yellowed, bathing won't make it better so limit baths to once ever 2-3 months unless they really need it, especially for Teddies because they can be prone to dry skin.