On Application Integration

I'm currently working on an integration for several applications for an organization. After some readings and discussions, i concluded that applications integration for my case can be executed for these three levels, which are:

1. Application level: this can be done in several ways like web service, or just simply put the application codes in one directory, like a portal
   
2. User Management level: use two levels of authentication using one pair of username-password used to access the root, or the home, next check if that logged-in user have the privilege to access the application listed in his home
   
3. Database level: put all the schemas in one database (this is actually because of the constraint existed in Postgresql -the DBMS used in this case). I need several application to access one type of data. To cut off data redundancies.

All in all, what i mean was, one object can be accessed in several ways by several other objects and viewed by several users in several formats.

regards.