hope the pond snails eat those hydra before they eat the baby shrimps.

As someone with shrimp only tanks and having to deal with hydra often, snails will not predate on hydra. You may see the occasional lucky munch, but overall even pond snails will turn away. Best method I've found is just doing the $5 panacur c fenbendazole method. Especially if you only have pest snails in there. You can clear the tank of hydra in usually 48-72 hours and are done after a water change. Shrimp safe, and pest snail safe.

To prevent hydra in the future, isolate new purchased plants for two weeks prior to adding them to your shrimp tanks, and also treat them with fenbendazole, as it will also kill off the other common pest planaria.