Coming from the skimmer design/mfg business...
The answer is YES and NO.
Up to a point, oversizing is OK.
But it all comes down to the transitional section and the neck of the skimmer in relation to the nutrient capacity of your system. Even a heavily stocked 20g system can't work with a skimmer with a 4-5" neck for example. You'd have to raise the water level past the transition piece into the lower section of the neck to reduce the reaction chamber volume enough to build up a head of foam/organics to skim decently.
Think about the best skimmers you've seen, the "nog-shots" people have posted online. There's nothing like going into a LFS and seeing a couple outmatched smaller skimmers on a 1000 gallon system, attached to a 5 gallon bucket of nastiness and the lid half cocked due to excess foam in the cup. Skimmers like to have organics to skim.
Having said that, going somewhat oversize is great for gas exchange and larger more powerful pumps are better at "lifting up" particulates to skim.
So... bottom line. Use the "Heavy Bioload" advertised rating when you're shopping and round up to the larger model when on the borderline.
