GNOME Office integration - recommended practices

Hello, 

i'm hacking on a small app, and would be very happy if you could give me
some advice how to achieve good gnome (and gnome-office) integration.
I'm trying to ask very general questions.

(1) Is it recommended for new apps to use an existing file-format as
their own native format?
If, for example, an application mainly deals with tables, should
the .gnumeric format be used? Should a diagramming app use .dia?
The problem is that while importing and exporting my own files wouldn't
cause a problem, my app of course could not read all files from the
original app - this would most likely cause confusion.

A possible solution could be to re-use the existing format, but give it
another filename extension. Then the original app could easily be
patched to recognize an additional filename-extension.

(2) What is the predominant clipboard format for gnome-office interop?

Thanks, 
Rob