Thank you for your post.
FileMaker Server is only serving the files. The file extensions for FileMaker Pro 7-9 files is .fp7, so you should be good to go.
The only potential problem that may exist is if you create a file in FileMaker Pro 9, and then another FileMaker Pro 7 tries to use that file on Server.
That said, you would be *very* well served by upgrading to FileMaker Server 9. It fixes numerous critical stability bugs, not to mention the new features it includes.