I'm not much of a rules gal, so I don't think we need to stay with knits only. To me, a top is a non-shirt item that covers your top half. Shirts are made with woven fabric and have buttons all the way down the front. We can include shirts here, too, if there's no shirt sew-along this year. I don't want to make it difficult for people to participate. I just want to keep us all motivated to sew!
There's no requirement to complete a top every month, either. We all have reasons for not getting one made in a particular month, and we end up making several another month. The goal is 12 tops in the year, but even that is just a goal, not a requirement. In my opinion, if you make one top, you've participated and succeeded in this sew-along!