One important consideration when plumbing in series, is that the very first tank (next to the UAT) takes a lot of abuse. When the engine is running, it is under suction..generally not a bit deal as long as it is full of fluid, but it will start to empty at some point.....BUT, when you are fueling, there is so much resistance in the fuel lines (depending on line length, number of tanks etc,) that the tank is under pressure...enough pressure that it can crack thin wall fiberglass tanks. (don't ask..)

I don't see any value in a crossfeed line unless the jet is held straight and level at 1 G. A passive cross feed depends on gravity to equalize the fuel level in the two tanks. With the way most of us fly, the fuel is always sloshing around and there would be very little pressure differential between tanks to cause any net flow one way or the other. Besides that, once the first tank empties, you are sucking air anyways. .. needless complication, weight, and potential source for leaks in my books.....