Okay most of its been covered. Most common smashers (N wennerae, G. smithii, etc) can do well in a 5-10 gallon aquarium. you can find . wennerae (also pretty active in most cases.) at tampabaysaltwater.com for 50$ including shipping. When full grown it will probably take out most of your cleanup crew, except large snails like turbos and maybe serpent stars and urchins. Personally though (And I'm kind of biased because I only had one smasher, and it was a boring G. chiragra, so yeah) I prefer P. ciliata. They are active, colorful (and prone to changing colors) and they will leave most shelled cleanup crew alone, though they may eat hermits. They need a 20 gallon, are spearers, need at least 4-5" of sand or some pvc tubing, but they will eat fish. But hey, thats just my preference. Also feeding, I suggest frozen food (shrimp, krill etc.) every other day, soakd in selcon or other food supplement once a week, and at least once a month, preferably more, live shelled foods. Also, read the molt sticky up top.
Dan