generally any cuke from the Holothuria species is safe for the tank. the only threat to wiping out the tank is from being sucked into a powerhead. the flesh of the cuke is toxic, so anything the eats the cukes flesh will probably die. that said i have had one in my 75g. it lived for about 5-6 months then something caused it to spit its guts out. this is their defense method, and sometimes they recover, more often they dont in our tanks. mine didnt make it. after about 2 weeks i presumed he was dead as he hadnt moved from on spot in 2 days, and wasnt looking very good. nothing else died in the tank. heres some more info on cukes.
Aquarium Invertebrates