Hi Welcome to Fish Lore
It's great you're doing research first. Here's a good link to start reading about setting up a tank and some other must know things for people just starting out:
Fish Lore Beginners
Be sure to read the links at the bottom of the page also.
A
DG should get along with cardinals, they're very pretty fish. For a nice size school and a DG, I'd suggest starting about about a 20g. You could probably get away with a 15g, but I think you'd be happier with something a little larger then maybe add a clean up crew like a few cories.
Good luck with your new hobby
