Canon 5D files can be linked via the Avid QuickTime AMA plug-in. Just make sure that none of the names or folders containing the file have illegal characters in it such as / etc. Also, if using the folder version of the plug-in, there is a limit as to how high you can go, and I believe the limit is 3.
For XAVC-S, the only AMA plug-in that will work is the one from Drastic Technology – MediaReactor. This is not a free plug-in but supports lots of formats. Other than that, you would need to prep the XAVC-S files first, before going into Avid such as EditReady, Adobe Media Encoder, perhaps even Resolve and transcode to Avid friendly codec.
Michael