I think an engelmann spruce back will make a much bigger difference than changing the bracing material on a rosewood back! You should be fine, Spanish cedar back bracing is quite common in classical guitars, as is mahogany in steel strings, which is even heavier. Considering those are the most common neck materials, respectively, I'm guessing the practice originated as a way to utilize all those wasteful neck offcuts
Depending on who you ask, back bracing material may be practically irrelevant. But I suspect that's mostly from people who make heavy backs that are designed not to move much. Spruce works for me, but I may try putting some of my neck offcuts to use one of these days as well. In fact, I may even try mahogany bracing on a mahogany top, just for the novelty.
I've also been thinking about trying western red cedar and redwood, for darker color against rosewood backs, although those do split more easily. Not sure it's a big issue on a back, since the only thing I'd really worry about splitting braces is hard percussive techniques. And of course dropping the guitar, but that never actually happens, right? ...maybe I should stick to spruce.
