What if I need to remind a person about an action I need from him (@Waiting) in a month or two. Should I keep this project on my projects list or at SM?
I'd put the action to remind in my tickler so I won't keep seeing something non-actionable in the Action Lists. If it's definite that you're going to continue the project, I'd keep it on the projects list. If it's unknown whether you will continue the project, I'd put it on S/M. If there's a decision point I'd put it in the appropriate place (a meeting to decide goes on the calendar; a follow-up email you need to send in two months, in the tickler file; etc.)