Heh, yeah. I really never know who'll jump all over an idea, and I do admit that I put on a few folks who were kind of the new kids on the block as well as a few that were just old standbys that I LIKED the character and thought they'd be pretty versatile. Liking vs. writing, though, turned out to be the issue. :P
More choices is never a bad thing in writing, really. Maybe just...offering a lot of options for how the person wants to do it? I mean, maybe you could offer the challenge like you did here (which already gave a whopping amount of options), or maybe say, "OR, you could only have 6 characters, and double up the prompts so that for a given pairing, you'd have two prompts for a given week....or I suppose you could do it promptwise and say for a given prompt "1 and 6 (for the short listers) or also maybe 4 and 11 (for the long listers)". Which er...might be what you meant. It might get complicated for you to work out in an easy-to-understand fashion, but really...if you say, "this is v. v. v. low pressure, this is just a random idea generator, just go with what sounds good!", then I think folks'd be game. :)