Juveniles will be fine. H.orientalis is a species with no special adaptations for feeding on fast prey, so they are not very good at catching healthy shrimp at all. They will catch the odd one but shrimp should definitely reproduce faster than the newts can eat them. Since you will need to quarantine the shrimp before you introduce them to your newts, and that takes aproximately a month, it´s very likely that if conditions are good, the shrimp you get will mature and possibly have their first new generation inside that period. So by the time they are ready to be introduced in the newt tank, you´ll probably have loads, and there´s no way the newts will be able to catch them all.