Is it possible to use Outlook 2007 with the rest of the Office 2003 suite?
Are there incompatibilities which will cause problems?
Yes, you can install different versions of different Office products simultaneously. The installer will ask if you want to replace the old version or install side-by-side.
For example, at work I have both Access 97 and Access 2003 installed to handle both older and newer versions of MDBs.