How does what Slim say about his pups (“I drowned four of ‘em right off. She couldn’t feed that many.”) How does this relate to the overall theme of the story?

This relates to the theme of alienation in the novel. The characters have no ties, no direct responsibilities, so it's easy to see how the life of a puppy (or four) holds no importance. The shooting of Candy's dog is another example, as is Candy's belief that he'll become useless with nowhere to turn.