When you buy a fish from a LFS
ask them (politely) to feed the tank. Then pick a fish that is readily eating. If they are too busy or won't feed the fish then compare tank mates. Skip the ones that have skinny bellies.
Most fish won't eat right away. I always feed the tank before adding any fish I feel this will lead to less aggression by the established fish. Then don't feed the tank for a few days.
If the new fish is not eating for after 3 days I would start with live food. Brine Shrimp for small fish. Also you could try garlic, it's supposed to entice them to eat.