Pfft, there is no such thing as overskimming, it's all speculation by those who understand niether the biology of corals, natural reef environments, nor the physics of foam fractionation (protein skimming). I'm not going to go into the details, but the natural reef is an EXCEPTIONALLY nutrient POOR environment. Corals have evolved their biology to deal with and live in this nutrient poor environment. We as aquarist should be trying to reach this nutrient poor state through skimming and use of refugia to reproduce the natural environment in our tiny micro-environment tanks. Furthermore, skimmers ONLY remove bi-polar organic molecules. They DO NOT remove vitamins and minerals the corals need.
There IS such a thing as wasting electricity through excessive skimming though. In an article in advanced aquarist online magazine recently, the authors quantitatively compared skimmer performance. Their conclusions found that protein skimmers are exceptionally good at removing organic wastes, and do so in a matter of hours upon the introduction of that waste. They further found that the limiting factor in skimmer performance is neither throughput (flowrate), air flow, nor method (venturi vs needlewheel vs downdraft etc), but rather that the limiting factor in skimmer performance was the volume of the reaction chamber. All the bells and whistles and claims mean nothing compared to the size of the skimmer. Indications were that you could certainly buy a skimmer that was so big that if/when bi-polar organics are introduced, the skimmer could remove them in a matter of an hour or so, then merely be spending energy the rest of the time doing nothing of use...
So where does that get us practically? Buy a skimmer that's reliable first. Second, avoid ones with small reaction chambers like the Redsea Prisms, Jebos, or Seaclones. Not only are some of those unreliable, but they're small and undersized too. And lastly, try to avoid skimmers with large in-efficient pumps. Downdraft type skimmers (more popular in the states) guzzle electricity for no good end, and some really oversized needlewheel pumps also gobble up the juice for little purpose.
So Trod, if the skimmer you have is a decent size, and is reliable, go ahead and just use it