We have a couple of details to help figure out what kind of snakes Grandpa might be collecting. While the most common type of snake in Westchester County is the garter snake, these creatures cannot digest the exoskeletons of insects. Since the snakes are fed flies in the play, we can cross garter snakes off the list.
It is likely that Grandpa is not collecting venomous snakes, nor enormous snakes, so the ones living with the Sycamores are most likely green, redbelly, brown, and ringneck snakes -- the Environmental Information Series describes these NY-native snakes "small, slender, and completely benign species that feed on insects, spiders, earthworms and slugs."
It is likely that Grandpa is not collecting venomous snakes, nor enormous snakes, so the ones living with the Sycamores are most likely green, redbelly, brown, and ringneck snakes -- the Environmental Information Series describes these NY-native snakes "small, slender, and completely benign species that feed on insects, spiders, earthworms and slugs."
Tangentially related, and very weird, here's a picture of a 1930s Burlesque dancer walking her pet snake: