I've noticed a few people on this forum with a similar problem, and it's putting me off horizontal dropouts. From what I know after 18-15 most people use 18-16 so I guess that would be the best way to go, but it would be good to hear from somebody running 18-14, I imagine it would be good for street/BMX style riding.
Anyway good luck, I hope you get it sorted.
RD