Greg, you can adapt a system later on, it is really simple if you need to do it, just better with 8gpm so that each guy has 4gpm each, running less gpm will not give you a lot of cleaning power, most of the vendors can sell you the green spring unloader and fittings you will need if you choose to do this later on.
Something also to think about is if you are running 8gpm, your 200 gallon tank when full will only last about 25 minutes or less, depending on how low the bulkhead fitting is and if you have water going into the tank at the same time.
Running 5.5gpm with a 200 gallon tank will give you about 36 minutes, depending on how low you take the tank and if you have water running in at the same time.
If you have some water flow and you don't need the 8gpm flow yet, you can make it with the 5.5gpm hot water rig until your business grows.
There will probably be a time when your company grows and you need two of the 5.5 gpm machines, a 300 or 500 gallon tank with hose reels and other stuff on the trailer, at that time it might be a good idea for a larger/ stronger van or truck to pull that trailer as when it is empty, it can easily weigh about 4000 to 4500 pounds with only 100 gallons of water in the tank. Until then the van will probably work with the smaller trailer and hot water skid.