RE: Concept Fleet Biplane
I agree that 21st Century is pretty heavy, and on 1/6 scale bipe I would not use it for fear of coming out tail heavy.
Koverall and dope can certainly come out lighter, but that depends on how much dope you use, especially colored. If you don't mind some of the weave showing through, it can be comparable in weight to plastic films. When I'm really concerned about weight I use Polyspan instead. It is very strong, but 40 to 50% lighter than Koverall. It is not a fabric--more like a synthetic version of silk span.
Those are some nice engine choices. Any one you pick has a counterpart that is a drop in replacement, so you can't go wrong.
Jim