I have kept peppermints with seahorses for 7 years. Never a loss. Have kept them with 4 different species of seahorse at that.
Yes, a small peppermint will be eaten by a seahorse but the larger pep's 2"+ or so will be fine.
Right now my seahorse is being kept with three peppermint shrimp. I do not worry,
As someone who has fed WC's ghosts for over 5 years at a stretch, there are many differences between ghosts and pepeprmints. IME seahorses will not eat a ghost shrimp over an inch and a half, even if it is a 10" WC who will eat small fish. 1" or so ghosts, and even some fairly robust ghosts can be prey to seahorses, somethimes they are even snicked into two and eaten in two bites, but a ghost of a certain size and a peppermint of a certain size will not become prey.
JME