Yes, they can. Of course, the complication is that the rosette has already been installed, and I don't want to get so enthusiastic about thinning the soundboard that I sand right through the rosette. I could just leave that part of the soundboard alone and do the major thinning below the sound hole, where most of the exciting stuff happens.
I think most of my builds were around .110, which is pretty much in the middle of what Cumpiano recommended. (.100 to .125). But I'm not married to that dimension. If John feels good about the .125 that came with the soundboard, I'm happy to go with that. My understanding is that the thicker the soundboard, the brighter the trebles are, while you start bringing up the bass range when you thin it. Since this is mostly going to be a finger picking guitar, brighter treble might be a good thing.
I've just prepared two mahogany shims to reduce the neck angle. I don't think they'll be too ugly, and with the neck angle issue resolved, I can proceed to build the instrument.