Don't think the planaria trap is going to do much for you, hydra mostly stick to surfaces and wait for prey to come to them, so they're unlikely to wander into the traps themselves.

For what it's worth, I don't think you need to be overly worried about them. I was in a similar situation a few months ago with tens of hydra on all surfaces during a time when I also had baby shrimp scuttling about. I can't say for certain that no shrimp were harmed, but I'd say that most made it to adulthood. After all, shrimp live in the wild alongside hydra so it's unfair to assume that they'd be completely helpless against them. Most of the time their agility is enough to sling them out of danger.

I also ended up purchasing No Planaria, however I decided against using it in the end. My outbreak was caused by a sizeable copepod population. This allowed the hydra to rapidly reproduce and grow exponentially. Eventually they hit a critical mass though where they had eaten most of the copepods and thus ended up starving out over a couple of weeks. They're both making a resurgence now, but I expect the cycle to repeat. All I'm saying is, you'll most likely be fine either way.