Good questions. 

I think everyone should post at a rate they are comfortable with and participate as much as they are able. I've seen a lot of good RPs killed by "I was waiting for someone else to post," which I'd like to avoid if possible. 

Roles aren't going to be dead set, it's a team of individuals first and foremost. To use your example, if one guy is the team marksman, it's not because that is a role that must be filled every single time, it's because that character is skilled in that area and assumes that task when it comes up. I'd like to see well-rounded characters, not minmaxing.