Globally replace all styles, variables and conditions?

Not directly. FM's format import is always additive. Nothing gets removed.

You can manually delete all unused syles from a catalog before importing. Just click on the Options button at the bootom to get the Set Available Formats window:

There was a third-party tool (CleanImport) that did this, but the site (Electropubs.com) seeems to have gone away (it's currenly parked at GoDaddy). If you have Framescript, then there's a script available in their library (https://framescript.com/?page_id=33597) called TidyImport that deletes any formats that aren't in the target document during import.