This is an interesting topic, and there are a couple points of view about it. Some people maintain that a skimmer should be on ANY reef tank, even if it's only used some of the time. Just as on large tanks, the thought process here is that the skimmer will remove much of the excess DOC's in the tank, keeping the water as pristine as possible (and as a side effect, increasing the O2 saturation as well).
Other people swear that a skimmer isn't necessary on a small tank, and that the weekly partial water changes that MUST be kept up anyway in small systems will be more than adequate to keep DOC's down.
I think what you should look at more closely right now is your lighting, flow, and heating parameters of the small tank. What lighting do you have that you plan to use to keep the corals alive? Your coral choice will depend on the quality of light you have pumped into this tank. Likewise, increased light may equal increased heat, so you need to make sure your temperatures can remain stable...this isn't always the easiest thing to do in a small environment. You may need to consider a fan blowing across the tank top, or possibly even a small thermoelectric chiller such as an ice probe.
Finally, what do you have in place for flow? A couple koralia-1's or the similarly-rated koralia evolutions would probably be your best bet.
Personally, I feel that religious water changes of around 3 gallons a week will benefit the tank more than a skimmer, but if you choose to there are a couple good nano skimmers available. If I were you, I'd focus on the lighting and cooling issues first, then move on to the skimmer.
