I work in a pet store and when people try to buy fish the same day they're buying a tank, I nix the idea. I always tell them to wait at least 24-48 hours before buying fish. That way they can have the tank set up and even come in to test the water to see if there's any harmful toxins in the water.
Even when they do come in to buy fish, I'd only sell them a couple if I knew it was a new tank. People tend to buy more than their tank can hold, and then wonder why their fish are dying!! I was told the general rule of thumb is an inch of fish per gallon!