Anyone confirm if i'm able to safely run FM11 on FM Server 10?

Depends.

If your files are protected with the new FM11 security feature: no.

Other than that, most new FM11 features are client-based so you're fine.

But if you do use new FM11 features (especially functions in field definitions) and you do something that would make that calc evaluate on the server instead of on the client then FM will return "?"

See the help file for when calculations execute on the server vs on the client.

Another thing to watch out for is if you use server-side scripts that use FM11 features: FMS10 will not be able to properly execute those.