I've been kicked out of a pet shop before 
I know what you mean about butting in, it's very difficult to know what to do. Probably better to speak to the shop manager about the advice they give, but they usually just trot out the "we've consulted experts" line, even though in some pet shops the terms 'expert' and 'salesperson' seem to be interchangeable.
P@H are really shooting themselves in the foot with their crappy advice, they have a terrible reputation when it comes to aquatics. I don't know much about other animals but I would bet they have a terrible reputation when it comes to rabbits/hamsters/gerbils etc as well.