First of all, Welcome! *gives a welcoming hug*
I'm sorry that people have seemed rude, I'm sure they weren't trying to be offensive.
The alfalfa hay is good, but I don't know about the oat millet. I have never heard of it being used, and I never trust anything a petstore tells me, so unless someone who knows for sure tells you it's ok, then I wouldn't feed her any oat millet. In case you don't know, here at Cavycages we never trust petstore info because the people who work their are very uneducated about the animals there, and do not take proper care of their pets as it is.
Here's a link on how to find out the sex of your piggie:
http://cavyspirit.com/sexing.htm
The baby should be seperated from mom at 3 weeks of age if it is a male.
Heres a link on how to do introductions/re-intros:
http://cavyspirit.com/sociallife.htm#Introductions
I hope this helps! Feel free to ask anyone anything on here, it is an awsome web site. learn something new every time.