I did spixis for a while. They ate some hydra, but hydra hide and they won't find them all. SInce hydra reproduce with budding too, they'll always be there unless meds are introduced.
Spixis are also illegal to ship across state lines and in IL there was some talk about them being illegal period. To be on the safe side, I got rid of them. Not worth being investigated or banned from aquatics for just one species.
I use .05g fenben per tenner to get rid of hydra. Takes about three days for them to ball up and disappear, but totally worth it. Never had a shrimp loss from it, but I use "fish bendazole" and not wormer for dogs.