Ron,
Yes, you can use pecan shells. I'd recommend wrapping them tightly in foil and poking a few holes in the foil so they don't burn too fast. Quantity depends on the meat: for a single whole chicken, I'd use one fist sized bundle (see above). For a whole brisket; I'd use 3 or 4 fist sized bundles. The results should be about the same as pecan wood which I use regularly. I don't use the shells because pecan is readily available in my area.
Paul
Paul