what kind of fifthwheel hitch?

He means that a gooseneck adapter on a fifth wheel stresses the fifth wheel frame, which is true. Some fifth wheels are designed to handle it and some aren't. My dad has had his Nash on a gooseneck adapter for over 20 years with no issues. I like goosenecks better than fifth wheel hitches, probably just because I've used them more, but went with the Andersen because I was a bit leery about using an adapter on my somewhat larger fifth wheel.

Having a gooseneck setup on your truck won't stress anything unless you install a gooseneck adapter on your trailer (which might be fine depending on the trailer).