I heard that hermits and snails were safe with cuttles, but I just put some in my tank with 1 Sepia bandensis and right away, the cuttle pulled a hermit out (didn't eat it, just killed it). Then the next day, she attempted to eat a nassarius snail that was munching on a silver side, but decided the silver side tasted better. And something killed another nassarius snail overnight. There aren't any predators in there except the one cuttle. I think she is just bored and is curious about them, but the poor little buggers can't take it. She leaves the other kinds of snails alone, but my nassarius and zebra hermits really get her interested.
It may be more that you have added something to the tank rather than what you have added. I have found the even a shrimp may survive an octopus IF it is in the tank before the octo arrives and IF it does not get overly curious. The octos learn quickly that anything I put in the tank is food (or anything new that get put in the tank, the I part may not be relavent).