Congrats on the new Degu! You should feed your Degu a mix of Chin/Guinea Pig pellets, I'm not sure about the gerbil pellets.. If you're buying the petstore Chin food with all of the treats you have to take all of the litle treats out and it will be fine to feed your degus. If you can find Chin pellets without the treats it would be easier for you
As for the new baby Degus, you might want to take it slow introducing them to the older one. Let them sniff each other through the cage bars first that way you can control whether or not they fight.
If they seem like they don't like each other (loud squeaking at each other) or if they're trying to fight don't put them with her at first. If this happens and you have a spare cage, you can put them in a seperate cage next to hers that way they can get used to each other, and introduce them slowly.
If at first (while your holding them) they seem to like each other and make they're grooming sound (sounds like a bunch of little birds chirping) you can try to put them together but watch them closely for an hour or so
If they get along well and you keep them together remember that Degus play-fight alot (standing up on they're back legs) which is ok but if they ever make each other bleed seperate them immediately.
Another snack that your Degu will like is Shredded Wheat (100% whole wheat, no frosting or added sugar). You can also try cutting up the carrots and leaving them out to dry for 3 days, that's the way my Degus like they're carrots.
Also, if you didn't get one already, don't forget to get your Degu a wheel so she can get her exercise!
Good luck with you're Degus!!