You can certainly have FileMaker Pro 10 clients running off FileMaker Server 9.
However, be aware of this known issue/problem/feature from the Read Me file for FileMaker Pro:
1.4. If a file is hosted with an earlier version of FileMaker Pro or FileMaker Server, and a password change is required at next login, FileMaker Pro 10 clients will not be able to use the dialog to change the password. This issue does not occur when the file is hosted with FileMaker Pro 10 or FileMaker Server 10. Clients can still change passwords by choosing File menu > Change Password, as long as a password change is not required on next login.
It is a good idea to read the Read Me files for both FileMaker Pro and FileMaker Server. Up to date versions are available at the FileMaker Knowledgebase.