Well, from watching my O. Mercatoris Einy (who was 1cm mantle length when I got him, about 2 inches long tip to tip), I'm going to say you shouldn't underestimate the ability of a small octopus when it comes to prey. He is very persistant when he's hungry, and if at first he doesn't succeed, he tries and tries again. I've fed him both male and female Fiddlers and he aproaches both of them with the same attitude. He usually waits until they turn their back, then pounces on them. Sometimes they are quick to defend themselves, and he has been pinched a time or two, but he always retreats, comes up with another plan, and makes another attempt. He's quite strategic in his attacks, using rocks and shells to help camoflauge himself until just the right moment.
I'm quite sure any healthy octopus is always going to win over your typical feeder crab. I haven't read of anyone having lost an octopus to a crab yet.