I am in a Ram 3500 but my 2500 had the same bed. I have not had any issues with the Andersen in the 3500. Didn't want the size, weight or hassle of a full size slider. I've read of some that do and don't use sliders in the 6.4' Ram bed so if you want a particular hitch I would then research it.
I'm not sure what the dealer was talking about with the 6.4' being the new "standard" bed and the 5.5' being a short bed. I guess I'm of the old school; 8' bed is a long bed and the 6.4/6.5 is the short bed. 5.5 is just.....too short.
The height of the truck bed does have an impact on the height of the pin/level of the trailer. The hitch and pin both have adjustments to raise/lower the respective parts to get as good a fit as possible. Unless your 2500 has been modified in some way you should not have any problems.
Looked at that trailer and that looks like a nice unit. What drive train do you have in your 2500? The reason I ask is that 10k is what the gvw of my last trailer was and it could put a strain on the truck depending on which Ram I drove, the engine, tranny and ratio.