The need for a skimmer depends of course on your system. There are a great many reef tanks that get along just fine without one, including mine. They are however nice to have around in case something does happen and you need help taking something out of the water, such as happens with coral spawning, or if a phytoplankton bloom needs to be cleaned up. Other than that, there is no real need for one. IF... your system is set up properly and stocked properly.
In your case, you would be better off with carbon than a skimmer when keeping mushrooms and zoanthids.
Chuck