There's not much different between a hip-width and shoulder-width stance unless you have some really strange proportions. A starting point is to take the stance you would use if you had to do a standing broad jump. Alot of people have terrible landing mechanics, so I wouldn't necessarily trust your landing stance as ideal. The only way to know for sure is to simply experiment a bit and figure the starting position wher you are the strongest.
For the question regarding a sumo stance and rack pulls, I've never seen anyone who had much luck with rack pulls helping a sumo DL. The problem is that the sticking point in the sumo DL is off the floor about 99% of the time, so rack pulls don't help that much. Simply doing moderate sets of 3-5 reps in the sumo DL is probably the best assistance exercise for it. You could also try box squats with a sumo-width stance or chain suspended squats with the starting position similar to your sumo DL starting point. If you want to do rack pulls as an assistance exercise for general overall strength, just use a conventional stance and start them somewhere around mid-shin.
