3/ Like someone else much wiser than me said: because these creatures can tolerate extreme conditions doesn't mean they thrive in them or that they can survive them for very long.
4/ I asked same question (about what NOT to keep) on another forum and got this answer from Puttputt:
"crabs, nudibranches that eat a specific food, snails that eat other snails, anything that you dont know what they eat, sea squirts, big star fish - they'll eat everything, big kina - as per starfish (my purple urchin consumed a live limpet (black footed one - you know) on the weekend, Sponges (they die and pollute), and thats all so far."