Hi Alan!
Others have granted you with ample and accurate information about your first question on resting the thumb. As a beginner, I can only talk about my very own experience. Personnally, I never rest the thumb on the rosette because I'm too afraid it would cause damage to the wood, since I have a long and very strong thumb nail.
Whatsmore, if I rest my thumb on the rosette, I have to open my hand further to play picado parts and the tension in my fingers makes it hard to keep the position repeatedly.
Finally, let me come back on the rasgueado thing in general since it is one of the most difficult part of flamenco techniques (but which part is easy, right?

). I read in a French method
Traite de la guitare flamenca by C.Worms and O.Herrero, that the rasgueado is not played (every time) on the six strings but in the middle. The pinch is given in the middle strings and the movement finishes on the first one. Once one says that, one could think that rasgueados are only a trebble string technique. And then, one hears Sabicas doing rasgueados on the bass strings... Depends on what you want to play!
I have understood very recently another important point, that still needs confirmation from much more epxerienced players : the brightness of a rasgueado is not in the strength you put in your fingers to play it but rather in the velocity with which your fingertips move on the srings. If you add both of that (ie strength and accuracy of position) the result is way brighter and stronger at the same time. I'm still practicing in not putting too much strength in my fingers and it is a challenging thing, because I realized that getting the rasgueado moves is one thing but adjusting the power in playing them is another!
