I would rather have one that is too big then one too small , JMO incase you decide to add to the live stock!!
We ran a 55 gallon for many years with out any skimmer at all , it depends on your habits whei it comes to the tank, I think a skimmer will save ya a little cash over time but IMO it should be cleaned regularly, The ones I have are
HOB skimmers so my sugestions are based on that style,,
JMO