You can definatly deep numerous species in a tank, depending on the species. There are some you are going to want to stay away from if you are just starting out, especially if you have a small tank, and definatly dont want to add more than one unless it is really big, and that is a LInkia or Fromia starfish because of their feeding habits. They pretty much only eat micro fauna and flora on the live rock, so that is why large tanks are needed (you need a lot of live rock). But keeping numerous species of others such as chocolate chips, brittles, serpents, would all be perfectly fine. As far as aggression, there are a lot of species that will eat corals (such as a chocolate chip) and some if they are very large will eat any inverts they can catch such as shrimp, although the shrimp are pretty darn good at getting away. As far as aggression towards other stars, it's not something that I would worry about. In the wild you may encounter that, as there are some species which feed primarily on other starfish. However nothing that is really available in the aquarium trade. As far as towards fish, the same thing goes with inverts. I've heard people claim that their large (LARGE) brittle star ate their fish. But the fish would have to be pretty small and very weak to get trapped. The fish is usually sick/dying before the star is able to catch it.
If you end up doing an entire urchin/star tank, I would try to feed each individual star and you are going to have to supplement the tank with algae sheets or macro if you can get enough of it for the urchins.
