correct in your case this should not be any issue as this is mounted like a regular ext4 which supports hardlinks just fine.

To know the backup scope, you can sum up all the apps listed there + email data + box data. Then you would have to multiple that by the number of backups which are stored (eg. once every day for 1 week or so) This would be the maximum of storage required using the tarball method. For rsync, it would be much less since it would do incremental backups.