> I will have a bunch of ideas that I have written down. Some of them are actions, but not necessarily things that I want to do right now, or additionally I may already have a next action written down for that project. Do these get put on the next action lists anyway? Or do they get filed in reference?
When processing your ideas (or any set of notes - meetings, travel, etc.) follow the processing and organizing workflow diagram for each one. What is it ("it's an idea for a bike with built-in magnets for sticking to cars for travel"), is it actionable (if yes: pull out the action, and maybe a project too if multi-step; if not: someday/maybe), what's the next action (break it down, e.g., research existing self-attaching bike patents), etc.
All the actions that you're committing to go on your actions list. Any related projects go on your projects list. If you decide to sketch out additional actions for each project (the planning process), put those actions in the project plans folder for the project.
> I am confused as to whether all actions for a project should be listed or only the next action
The minimum is *one* action "pulled out" for each project. This ensures every project is moving ahead. You can pull out more if a) you want to move it ahead faster, b) they are independent actions, and c) you can handle the additional workload. One's often enough, because you'll have one from *every* project, and you'll have between a few dozen and a hundred of them!
More here, FYI: Dealing with Meeting Notes - GTD to the Rescue!
http://ideamatt.blogspot.com/2005/09/dealing-with-meeting-notes-gtd-to.html