Starfish are very fragile creatures, should only be kept by experienced hobbiests, in an established tank. There are some kinds that can be spot fed, but many popular species will frankly starve to death as they cannot be properly fed in the aquarium environment. For more information please do a search (click on the button--top right) and particularly read responses by one of the moderators nic-named Ophiura. When it comes to starfish, she's the resident expert.
As for skimmers--not necessary, but a really big help. I wouldn't run my tanks without one.
