I just wanted to take a minute to share what I know about this subject.
Does not have the ability to shrink period. It can only grow. The only possible solution is to do a file backup using something like xfsdump, then recreate the filesystem. Or make the smallest partition possible when you first install, then grow after imaging.
There is nothing special about fusion drives, the issue is that fusion drives are coupled with OSX’s core storage feature. Core storage is basically lvm for mac. When using a fusion drive a volume group is created and spread across the two drives, giving you added speed on the ssd portion. I assure you the drive is not hidden and you still control over your drives. It just appears as one drive. You will most likely never be able to image core storage from a linux environment except as raw. It’s like when you want to capture lvm on linux you need to active the lvm first or it will be captured as raw, the only problem is there is no way to activate core storage on linux.