I use it to keep the internal resistance down in the tubing so the tanks do not swell during fueling/defueling, also it makes it easier for the pump to draw fuel through all of that tubing. I am running a RAM 500X. My ducted fans have a hard time sucking through standard tubing so I would go with the big stuff, there is probably a chance of a flameout if you use the small tubing, also get yourself some aluminum fuel tank caps, they will not split like the plastic ones.

Chris