I would never use a snail to clean up anything; they have such a massive fecal output and produce so much bacteria that they'd probably make your fry ill. From what I've gathered, fry are very prone to bacterial infections, so I think it would be a bad idea to try the snail in with them. One of my adult bettas kept on getting fin rot, and I firmly believe it was from the snail he was in with; darn thing produced a ton of waste. His fin troubles left when the snail did.
As for the shrimp, I'm not sure. Are they carnivorous? If so, I wouldn't suggest it as they might munch on the betta babies.