The good news is it isn't a big investment to try it. 42-17 is a pretty low gear already, freewheels are cheap, $20 or so, they're easy to swap, the tool is like $5. I have a surly set and formula set and you could run them flip flop with freewheels on both side. I say try the 18 you should be able to do it with no worries about chain length. I think that's a pretty small gear for the flats, but if it's rolling everywhere you go it might be just the ticket for you.