Echoing one comment above, I'd be surprised if using 0.18-0.59 would damage a Martin guitar with the strings tuned down as low as that Carthy tuning mentioned above.

But I'm also not sure that they'd be hugely slack either. I say that based on the fact that my gypsy jazz guitar is currently tuned down a whole 3 semitones, to C sharp (though still in standard tuning). I'm using a set of strings that are 12-50. Frankly, they don't feel as slack to me as when I've played using light gauges in standard tuning.