There are many ways to implement your assignment. So if you think your desicion is correct, then you must do that. In my code I also used only my own interface and not DBMain interface to retrieve data from the database(local or server).
I implemented 3 classes which implements Serializable interface. I used them everywhere I need. It doesn't mean if you use a serializable object without calling any RMI code it is serialized. Serializing is only done by RMI protocol when that object must be send or received by using by RMI server.
I hope my answer is clear. My english is not so good