The un-finned smooth 1509 is the largest motor you are going to be able to run in a Stampede/Rustler/Bandit/Slash. Anything longer and you are going to have a real problem with the wires exiting the can hitting the tires. Even at that your gear ratio is VERY limited. I would look into a Medusa 70mm length as thier wires exit form the size vs. the end.
A finned can neu will fit the flm tranny case, bit you will have limited adjustment. I ran a plett maxx which is similar in diamter to a finned neu. I used a 58t jato spur and a 15t pinion and it was fine. You will not need a 1515 motor in anything 2wd. I am sure that a 1509 will be plenty.