I've had all types of hermits and even the scarlet with yellow face hermits still attack my snails for their shells. I also feed my tanks plenty, so its not like they're starved. They also like to sit in my zoo colonies and irritate things. So no, I'd rather put a good variety of snails in than hermits. Or just some really smart snails if you're going to mix the two.
Pufferpunk one of my customers had trouble keeping sea hares alive in his tank as well. Turned out he had stray electric current in his tank and didn't realize it. This might be your problem.
I think it depends on which kind of sea hare you're keeping, all of the sea hares
[FONT=Arial, Helvetica, sans-serif](Dolabella auricularia)[/FONT][FONT=Arial, Helvetica, sans-serif][/FONT] that I keep are very hardy except for the occasional one getting sucked into my tunzes.
When I get new ones I just drop them right into the tank, they don't seem to be at all sensitive to the change in environment/water.
Some people sell certain hares that are more delicate than the ones I keep. For example I bought these 2.5" ones that resemble a smaller version of a large sea hare that occurs in our local waters. I bought a bunch of these guys. I lost a bunch from sensitivity and then the rest wouldn't touch any of the 5 types of algae I tried feeding them. ???
So don't give up, if you get the right ones they will do a great job. Mine are taking care of a bad brown slime algae problem as I speak. ;p