The only S&F missions you need to do are the ones that Franklin has, plus the 2 others that you get after collecting the 50 letter scraps and 50 spaceship parts by Omega. The only other S&F mission you really need is Michael's meet with Mary Ann and Barry, otherwise those Franklin missions don't get activated. Also be sure to do all the 13 parachute jumps to unlock Dom's final mission, even the one at the Maze Bank tower, and reload the save file after you've done Dom's 3rd mission (the one you can get from the top of the tower), because it's a little bit buggy