Is it possible? "Yes" but if it is advisable that might be an iffy "Maybe, it depends".

 

I've used the latest and greatest Smartermail as Incoming and Outgoing Gateways when my primary Smartermail server is a major version (or two) behind, but never that many versions behind (I know some major things happened in v8-9 of Smartermail and the WebSvc calls have changed somewhat so I'm not sure if Smartermail Gateway Mode and Web Service User Verification would work between that new and that old of versions.).

However, if you are just using Backup MX or Domain Forward (with Web Service User Verification and Smartermail Gateway Mode both disabled) then I don't think you'll run into any problems. In theory your Backup MX shouldn't care if your primary is even running Smartermail, so it should work perfectly fine.