My little G. Viridis has a few tankmates that he doesn't bother.
I have an emerald mithrax that I introduced to eat some hair algae (the little bas*^rd also ate my grape caulerpa, but ,oh well...).
He has been whacked a couple of times when he tried to steal the Mantis' food, but otherwise he is OK. Of course, he hides 90% of the time in the far back corner, only to rush out when the 'magic nori stick' comes calling, then he rtreats again.
The Baron (my Mantis) doesn't seem to like hermits much, only having eaten 3 or 4 in 3 months...he ate a few of the smaller ones, but otherwise. he ignores them.
Snails, on the other hand, are eaten pretty quick, except for the larest ones (he's a little mantis, at 2.5" or so, and not liely to grow much bigger...). he eats the smaller ones with relish (gusto, not a pickle condiment), and will tackle a bigger one occassionally.
I have a colony of pink star polyps on a piece of LR that measures about 4", he has never messed with them.
I have a small feather duster which he has also ignored.
I have a colony of red mushroom corals on a piece of LR tat is about 3" around (time to learn how to frag them, they are getting huge, and crowded...).
LOSSES
I added 2 green Chromis a while back in a 'can they live together' experiment (Yes, I am a newbie...). All went well for 3 or 4 days or so, he ignored them. Then, one fishie got curious abd ezplored that cool looking cave under the big LR Castle...he met a quick (well, he swam around a bit before he died, but it was horrible to watch) death. I returned the other to the LFS.
I had 3 Xenia polyps(stalks?) on a piece of LR. they have disappeared, though that could have been because of a small group of aptasia (too bad he doesn't hate aptasia, folks would love mantis then...) on the same rock. I am still figuring out what to do with them, as i love that rock and it is isolated from others (no spreading so far).
I also have a tiny pistol or mantis shrimp that hitchhicked in on that same piece of LR (I saw a blackish blur once, and hear him *pop* after the lights go out...). So far, he has evaded the Baron's wrath, though i am going to get him outta there if I can ever find him.
This is all in a little 6 gallon tank.
Hope that helped.
-Ron