Ok, what's done is done. Next time you'll know better to adopt. Now it is time for you to take proper care of them.
Search the links I provided for sexing adults and the baby. The sooner the better, it doesn't hurt for Chessie (sometimes you can not tell the difference until you push upon the sack), and don't wait till the 3rd week for the little one to see what he/she is. You can handle the baby from his first day, don't worry.
For food, we highly recommend the best two brands, not any "special food" that they might like: only Oxbow (that you can order online or find in stores), or Kleenmamas (just online).
Oxbow Animal Health | Home Kleenmama's Hayloft
Please make sure your C&C cage is big enough, no pet store cage please, they are way too small
Guinea Pig Cages, Your Cavy At Home
Feel free to ask any other questions.
On Wallmart, you mean this?
Walmart.com: North State - Superyard XT Portable Playard and Gate: Gear If yes, way too small. You can make your own play pen from grids with zip ties.