Is it possible to automatically set the ACL for the first page in a new namespace
I'm not aware that this could be done automatically.
The start page should be visible to all. The other pages will have their ACL set according to who should see them.
You need to set an ACE for the directory and another for the start page. When access is considered and ACE on the start page will trump over the directory's (namespace's) ACL.
Or would it be easier to have all the "start" pages in their own namespace with links through to a per document name space?
I suspect that this will be much easier to maintain.
Can ACLs automatically be set on a page when they are created ?
This is a repeat of the first point. You could set up a simple script to do this for you but otherwise the pages will be within the namespace and use the appropriate ACL.