I think Rex 🤝 Kallus friendship is neat for a lot of reasons. Firstly, I think Kallus needs a role model for deprogramming imperial behavior and would not, as former ISB, be very easily trusted amongst other defectors. Meanwhile, Rex has a lot of experience with separating what's him from what's "programming" and would probably understand more about quitting the thought police than he's truly comfortable with. Lots of uncomfy realizations abound for both of them when they compare notes, you know?
But also I think Kallus would be the guy who appreciates the boring stories from the Clone Wars, the stories about Rex and Cody doing the admin work that became the framework for imperial codes later on, the guy who really just gets it when Rex tells the story about him and Kanan sneaking around disguised as troopers. "The commissary" is like the joke they end up sharing or something and that's a fun little way for them to get through after Kanan is gone because it kills Rex to lose another Jedi friend and Kallus has zero experience healthily mourning and I think that would be nice for them to have a shared thing that's bonding but also healthy grief.