sea stars should be fine, try a damsel to see what kind of temperment your mantis has, if it is cool with a damsel for a while, you may be safe trying a clown or some nonaggresive (non crustacean predator) fish. My peacock lives with a LARGE turbo snail, choco chip star, bubbletip brittle star, large Allardi clown, a few hermits, and tons of "snorkel" sand burrowning snails i found on ebay as food (but she wont eat them!); this is all in a 26 gallon. But my friend keeps his Oerstedii in a 10 G with two saddle back clowns. The second clown was just recently introduced after several months apart (we split the pair into my and his tanks last year) upon introduction the newbee (smaller) clown got his a$$ kicked...BAD...so we put the resident clown in a time-out tank for about four days, upon reintroduction the aggression had subsided a bit, but still was not lookin good; even the mantis was coming out and investigating this "intruder" so we moved some rock around and made a new little protective corner for the newbee. After about a week, everything is gravy, they clowns look like they are flirting all the time! swimming and dancing together and i think the mantis is more scared or cautious of the clowns than the other way around! anyway, point of the story is that it can certainly be done with a nonaggresive roomate and a laid back mantis.... I am about to redesign my wennera set up and upgrade to a 12 nanocube, and am thinking about having two small aquacultured seahorses since they mind their business, and my wenny lives with a firefish and has only killed a couple snails in 8 months....