I don't think a skimmer is necessary for a smaller tank 40g and under, but even that depends on what you're trying to keep. I have a 40br with maybe 25-30lbs of LR and a 1inch SB. Fish are a pair of percs, pair of wrasses, a canary blenny, and a royal gramma. I have a huge RBTA, torch, froggies, purple cap(yes its purple), ORA green birdsnest with GREAT color, digis, and the rest softies and LPS.
It just really depends on what you're trying to accomplish. A full blown SPS tank would never work with my set up.