I have a reef octo extreme 3000 and I would not go with any other skimmer except for tunze but they have all nano skimmers. The reason why I say the reef octo over the tunze is the parts are cheep to replace and easy to clean.
When you have a smaller tank, your usually better off just doing water changes to maintain the water. Skimmers are not really cost effective. If you still want one, any of the above would be fine. On my 30 gal tank, I use am AquaC Remora, which I like a lot.
Surface scum is a problem IMO. It restricts the exchange of oxygen that is necessary. Weekly coffee filter skimming wouldn't cut it in my book so whichever model you get I think it's a good idea.
If your getting surface scum, you should be able to break it up easily just by using a powerhead to agitate the water surface a bit. These are usually bacteria films, and not a big issue of you clear them off. You'd really have to have a thick scum to interfere with the gas exchange.