Can I install FrameMaker 12 on Windows 7 with FrameMaker 9 and use them interchangeably?

Yes – you just have to be careful about file associations & versions of fm files (so that you don’t accidentally upgrade an FM9 file to 12).

Also keep an eye on Acrobat Distiller (if you have it) – only one version of the distiller can exist on a workstation – it doesn’t play nice with multiple versions of itself.