Just do exactly as you have stated. Plum your auxiliary tank into your existing one. As far as what we are running dont worry about it. Most are using the stock tank. Just get what you can afford and ones size that will fit your needs and bed and bolt it down
do you already have factory dual tanks? if not, that would be a cheap place to start with junkyard parts.
regardless of what you have there, i fear that a straight gravity system would result in fuel overflowing out the factory tank's filler and/or vent, so i think you would want to have some kind of valve on there to only transfer when needed, probably manually