I know you asked this because Cedar is so rot resistant. Here's what you need to understand. Wood does not rot unless it is exposed to outside contaminants. IF you do a good and proper encapsulation of the wood and nothing happens to Break that protective barrier it will NOT rot. Doesn't matter how "BAD" the choice. The other consideration when selecting wood should also be it's structural Strength. Cedar does not have a very high rating. Most softwoods don't. Hardwoods do. The reason most people use plywood instead of dimensional lumber is...The RESIN that holds the wood laminations together. This gives the plywood it's strength Bottom Line...use ext. Grade plywood for all your wood pieces (Unless they're for cosmetic purposes) and concentrate on doing a Great job of fiberglassing.
