we use primarily chicken here, little beef and ALMOST NO pork. im with reinhard, I used to debone thighs and use all the skins and fat, then i cut out skin and now trim off fat too. I've found for my family and me, we can just buy deboned chicken thighs, grind and use as is. no adding fats. for us, plenty of fat in meat.
now turkey, never used it for sausage so i can't say much accept experiment with it and update us. i don't think i'd add any additional fat. my suggestion would be to use meat and turkey fat from the bird, up to you as to use the skin or not. see how it turns out with meat/fat n no skin. or better yet, break it all in half. half with skin, half without.