Talked to @Pri on Discord and got these answers:
Question 1: use XFS for disks in the unRAID array, it's tried and true, reliable.
Question 2: Don't set your shares for files to use the cache drive until after you've migrated all your data, it will be a lot faster and stop your cache from hitting 100% capacity before Mover is invoked. Also SSD's have a finite amount of write cycles so it's a good idea to leave them out of the initial onloading of your data from your previous system for longevity too.
Thank you @Pri