I have been trying to break up my programs into an activity and a corresponding xml layout for each view. If you have one activity and all those layouts, you have the potential to have a monster block of code in that one activity. I find that breaking it up makes it easier to read and debug.
As for prompting the user before initializing activities, i'm not entirely clear on what you mean. You need to load an activity before anything happens, in your situation it could easily be a simple password acception activity. If you're talking about passing information between activities, you can package data in an intent, and use that to start a new activity. Then in that new activity pull the information out of the intent.