I make tamales twice annually - have been for about 12 years. I would not use pulled pork primarily because the pork should not be completely cooked when you roll your tamale. I don't mean the pork should be pink in the middle, rather it should be over 160 degrees (done), but not in the fall apart stages. You are going to steam these things for a couple hours so you want something that will eventually be fork tender, not something that already falls apart.
I also would not boil the pork. To me that's just like boiling chicken or ribs before Q'ing. Cube up fresh pork butt, add your seasonings and cook in a stock pot until you are almost fork tender, then add your chile verde or chile colorado.