If your talking about a reef tank with very little bio-load, such as a softy tank and a very few fish, it is easy enough to do with without skimmer or filter.
Just that the more bio-load, the more often you need to do water changes.
My 3 gal, I have 4 nice colonies of favias, approx 4 lbs of rock, an anemone shrimp, peppermint shrimp and a tiger goby in it. No skimmer, no carbon, just a filter pad, and weekly 25-30% water changes.
I would think that you could easily do an SPS tank, even with a few small fish with a HOB skimmer and filter. Like Mojo said, just have to do water changes a little more often depending on how much you feed the coral and fish.
IMO there is no reason you cant do a full reef system with that equipment.