Man, that 18" wide kills a lot of your options... Ever think of trying to trade for a wider tank?
If you're definitely going to keep this one, probably tats/scombs. Not sure what smaller ATF you could go with, as most would be pressing up against that 18" width to turn no matter what. A school of tats would be awesome, though. I think they normally top out around 12-16" (I haven't personally seen any over 16" ever, and not many over 14" in an aquarium setting). Maybe a school of tats, falcatus (red tail barracudas) or odoe pikes? What kind of characins do you like the best?