They may look cool but can do a number on a tank and once they establish themselves it is incredibly difficult to get rid of them.
Hard to really say with SW (tank size, lights, etc.). A lot of it has to do with your personal budget. The bigger the tank the better as the water chemistry remains more stable, but larger tanks generally require much more pricey equipment to support them. A decent-sized protein skimmer, for example, can cost several hundred dollars, and that isn’t including the sump system you would need to put it in.
I don’t want to totally hijack this thread. I’m relatively new to this site but if it has a private messaging system hit me up and I can discuss the basics of what is needed and what shortcuts you can take and still have a healthy sustainable setup while keeping costs low.