Yikes...two 'controversial' questions. Here's my take on it:
1) Is RO necessary? I think the correct answer would be that adding anything other than pure water to a marine tank is asking for trouble by threatening your tank with algae blooms, heavy metals, nitrates, phosphates, etc. The means by how you obtain it is up to you. RO/DI water is the purest that you can add to your tank (Aquafina and Dasani are both RO but minerals added). IMO, pure water is the only thing that should be used. You can obtain it:
-by buying your own RO/DI unit and making it
-buying RO water at your lfs
-buying premixed SW at your lfs
-buying bottled ocean water (and it still may contain nitrates)
-using bottled distilled (may contain copper if made from a copper distillation apparatus
2) Skimming is an extremely controversial area when it comes to nano tanks. In mini-reefs they should be standard equipment. In nano tanks, some skim, some don't. IMO, 10-20 gallons and under and you should be fine with consistent 10% water changes. 20 and over it is controversial. I have tried skimming on my 24 and to be honest...I haven't noticed any difference and I rarely skim unless my tank seems to have a problem. Others swear differently. It is a personal choice based on what you are going to keep in your tank, the volume and your budget. EG...if I had a 30 gallon nano with SPS and fish, I'd skim. SH