Ive never heard that. I've heard that if the skimmer is to big that it can pull a lot of the essential elements and stuff out of the tank, but never that it wouldnt work at all. That goes against the theory that if you get the right equiptment in the first place, when you upgrade the size of your tank you wont need to upgrade your equipment
You're not going to overskim with a coralife skimmer. I still haven't seen one person overskim a reef..........whatever that means. Removing minerals etc is a lame excuse since water changes will replenish anything lost, and a skimmer will not remove anything to create an imbalance anyway.
I guess in theory if the tank is really small and you use a super industrial strength skimmer, its possible for there to not be enough crap in the water to skim out on such a large level, hence no foam. But I dont think the smaller skimmers that we all use are going to be too strong for our systems. If there's no foam, its probably because it removed most of the stuff out pretty quickly and the system is in good shape.
I thought overskimming is when you remove potential food for corals. I.e. zoo and phyto plankton.
The theory I read about a skimmer being too large to work, was that the neck of the skimmer will be too large in dia for the bioload. The foam "head" will never get enough "dirt" to crust and flow over the top in to the collection cup. Meaning you will not generate any skimmate.