You can use just about any type of shotgun to shoot skeet, as long as it can shoot doubles. Pumps are very uncommon in the sport. The bulk of the guns used to shoot skeet are over-unders and automatics. The unique part of skeet is that competition is shot in four different gauges. The answer is no.
Some companies specalize in making what are called tube sets.
If you have an over-under you can send it out and have a system of interchangable tubes made for each gauge you wish to shoot. These tubes are the full length of the barrel and just drop right in and you are ready to shoot. Most models are made so they make the gun weigh as close to the origional 12 gauge as possible. Briley is a compay that makes custom tube sets to fit most 12 gauge over-unders. Use the links below to visit Briley and the Italian gun maker Beretta.