MANDARIN GOBY PAIR eating brine shrimp?

My LFS currently has a pair of these in hist store they have benn eating brine shrimp for the past month. I want this pair! is it possible to keep them eating this or will they just eventually die. I have a 110 gallon tank with about 100 pounds of live rock and a deep sand bed, but i have never been able to see copepods. I dont want to buy them if i am going to just kill them. Can they survive if they are eating prepared foods? will m,y sebae clown bother them? will any of my watchmen gobies bother them?