I think it all depends on the bird. Some will do fine with a single female, even with chickens around. Others, want every female they are with as theirs. I would get a female, as long as you are VERY comfortable with sexing (if you ended up with 2 drakes, all bets are off). Then, have a back up plan, if needed, and seperate the ducks from the chickens.
My drake leaves the chickens alone unless they get to close to something he wants and then he grabs a tail feather. He does have 4 girls of his own but has been down to 1 when I had 2 setting and 1 raising her babeis. He still didn't bother the chickens then.
Oh, I'd also avoid large breeds, to make things easier on the runner drakelet. If he has problems mating with, say a Pekin, he could turn to the chickens. If you can get a small or medium breed, I think that would be better.