The sand star is not competition for the other two, but the other stars are significantly more demanding. Due to their specific dietary needs, both the Red Sea or Fromia need a well established tank of 100+g. I would be wary of a two spot goby in a 35g tank. Though they are small, they can really burn through microfauna in a aand bed. Also, I have found lemon chromis to be pretty territorial, and would not be a good fit with the other species in a 35g tank.