I would definitely give the gun store a piece of someting if it told me that I could use other barrels. Did you buy the gun and the barrel at the same time or from the same place?
I recently bought a Remington 870 Express synthetic 7-round 18.5" for home defense. I also wanted to shoot skeet and trap etc, so I bought a 28" vent rib barrel made by Remington from cabelas. When I talked to the guy from cabelas he said that it should fit all 870 models, but it seems to me that it should only fit the 5 round guns.
You are learning the way most of us have, which is to buy your first gun. Many, if not the majority of people working in gun stores don't know as much as they think they do. That is probably true about most of us and myself, but they hold themselves out as being knowledgeable.