A short bed is usually 6.5 feet and that's enough for a fifth wheel. Just be careful the first few times when you back up and turn - depending on just how where your hitch is mounted and the style of kingpin on the trailer, it may be possible that the front of the trailer can hit the back of the truck cab in a sharp turn. Watch carefully until you are sure you know just how far the trailer can come around in a backing turn. [Going forward will never be a problem].
Any 2500 diesel should be able to easily handle a 7000 lb trailer, even if the 7000 is only the dry weight. You should use the trailer GVWR to evaluate towing capacity, though. Sooner or later your trailer will be loaded to its GVWR and that's when the truck really has to be able to grunt and handle it.