you could get a dwarf lion. i got my 120g tank for the purpose of getting volitans, and at 18" wide i felt it was too narrow since they can get 15" long and 18" wide from spine tip to tip. You also won't be able to keep a yellow tang in the tank. Tangs need lots of straight line swimming otherwise they will get ich and crash the tank. The other fish sound alright, i have gold striped maroon clowns, flame angel, pajama cardinals and they get along great with eachother....well my female clown chases off the flame angel once in a while,but usually the Niger trigger keeps everyone in check.
I hardly ever use my skimmer, once or twice a week for an hour or so. so it's really up to you what you wanna do. Also, filters are up to you. i really only use mine for flow (oh yeah, you're gonna wanna buy powerheads as well). The main filtration you're going to wanna use will be Live Rock with some Live Sand if you choose.