Re: Categories
If you look at the "Manage Permissions" page (tiki-objectpermissions.php), you'll see there are a number of permissions under "Category". I imagine not giving regular users "Can create new categories" but giving them "Can add objects in the category" would do the trick.
Was there some difficulty finding the permissions page and the specific permissions, before you posted here? (The procedure should be fairly self-evident so I'm wondering how it needs to be improved.)
-- Gary