If your girls accept her and you have space for three pigs (5ftx2ft is the minimum for 3 pigs) then there are some things to consider.
Are you absolutely and completely
100% certain that the 6 weeker is -
Female.
All her cagemates since the age of 3 weeks have been female.
Has not been in contact with a male, even for a few minutes (father/brother/non-related - the pigs don't care).
100% healthy - no signs of skin/eye/nasal problems on her or her cagemates or other pigs in household.
Otherwise you will need to -
Quarantine her for 2 weeks to ensure she is not carrying disease/parasites. Though fungal infections can take 6 weeks to show.
Place her on 10 week pregnancy watch.
Have the time,money and space to house any babies she may already be carrying. You may also have sadness ahead if she is pregnant as not all births go well for mum or the babies (approx 20% have life threatening complications).
When getting pigs from a non-rescue source it can be a 'lucky-dip' as you just never know what else they are bringing with them. Hopefully this little one is happy, healthy and not pregnant so she can get her happy ever after with you.
This link gives good info on sow behaviour so you can know what to expect from your girls when they meet her.
http://www.theguineapigforum.co.uk/showthread.php?t=38561
HTH
Suzy x
