W1S1 is less about rules than goals (and guidelines), imo. While officially the challenge is to write a story a week or month, the organizers have emphasized flexibility according to each individual writer's schedule and time.
Which is great, because for the challenge to be personally successful it needs to have some kind of end-game. If it were just a 'let's participate' thang, it'd be very easy to go 'woohoo' and then drift off after a few weeks, without much of anything having been learned or gained.
For some the ideal goal is a story a week, for some a story a month, for other two a month, or maybe it might be four one month and then one over the next two months. Or something else entirely.
I reckon as long as you set short story goals that will challenge yourself, but still be attainable, you're a W1S1er. Even if that goal is to write 2 short stories over the course of the year. For you, depending on your circumstances, that may be a real challenge.
If you decide you'd like to join in, but figure the muse will play hard-to-get if you try to chain it to some goals, that's cool. You can still be a W1S1er. I bet by hanging out with us your productivity will rise, if only thanks to positive peer pressure.
And even if you miss a month or two or three, that's all good. If you pick yourself up and push on you'll get there in the end. Writing's a longterm thing, and challenges like this help us build our writing stamina.
Write hard, peeps!
p.s. but if we do have to have rules, then a drinking game of some sort is definitely in order...