stock is 15/66, you'll have a very hard time stripping the clutch bell with a nylon spur gear

Yes,
a MIP clutch is a spring style 2-shoe clutch (works way better than the stock (no spring) 2-shoe set up.

Can't help ya with the .18,,I only race so .12s only for me
I'd guess move up 1 or 2 teeth on the bell at least