Can I use the management users portal to manage the accessibility of each portlet?
YES: You could restrict the access permission on portlet window. Just trigger the page/layout edition, then click on the pencil icon to edit the access permission of portlet window.
Can a Portlet to show different things depending on which User is logged in? Ex: A portet box containing a text editing, if "admin" show 5 options, if "user", the same portlet show only three options. It is possible to do this using the database own portal? Or do I need to have other portlets for each type of User?
YES: GateIn provides an API for authorization
In a large project, which has sundry portlets, it uses a project with sundry portlets (sundry package) or "just" sundry portlets projects?
I think the module organization should base on the buisiness value of portlets.
Where do I found the .css file of skin: "simple skin"?