The IBC tanks by themselves are really flimsy, they need a very good cage.
My own tank is a 650l one, the walls of the tank itself are 3 or 4 times thicker than those on an IBC
I have a 1000l IBC static tank, and was really surprised how thin the walls are, I understand now what they have such a comprehensive cage.
But the cage is also part of the pallet, so why would you need to fit another pallet underneath it in a van
The 1000l tank will mean you will at times be way overweight, no matter that you say you don't intend to fill it up, the odds are that you will fill it up far enough to take you past the safe limit.
I am glad that he cage my tank came in allowed me plenty of variability when it came to bolting it into the van.
The fuel tank was underneath, but I was able to pick and chose where I positioned the plates and bolts and therefore avoiding the fuel tank.
Ian