I wish I could answer your question but I honestly have no idea, I like larger sprockets on my single speed coaster hubs, instead of the normal 18T I like to go with a 22-24T.
If you're building a shifter bike with a shift kit you could contact Pablo or someone else at Sick Bike Parts and most likely they will be able to tell you what is best for the set up.
15T seems small to me but I dont run multi speed hubs so that may be a good size, I just dont know..Shrug?????