I ran a skimmer in a freshwater tank (180 gallon with 2 Oxydoras niger) and had it setup to dump into a wet/dry filter. The skimmer never "skimmed", but I thought it must have oxygenated the water well.
I did it just to try, I had a Berlin skimmer sitting around. Pretty inneficient way to do things, I'm sure.
A protien skimmer is designed for saltwater setups due to the water's density. Protien skimmers remove protien wastes before it has the chance the break up into ammonia. A freshwater system does not need one. All they'll do on a freshwater setup is oxygenate it to a small degree.