Well first, rear surrounds are part of a 7ch system while a 5ch just uses surrounds. Just clarification for moving ahead.
So the def tech in walls you linked would be fine but having driver sizes that fall in between certain sizes is unnecessary. It only matters how they perform. For placement I would place them in the side walls rather than the ceiling.
But I just reread your above so it sounds like you have to go into the back wall? They will
still be connected to the surround L/R terminals, not the SBL and SBR. If you’re only looking for a traditional 5.1 system, they can go a little higher, like maybe between 5 and 6 feet, depending on how wide the room is.
If you’re intending to do Atmos in the future, you’ll want the surrounds to be just above seated ear height. I have never found aimable tweeters to be useful, so I wouldn’t put much weight into that.
Personally I would also consider a subwoofer upgrade, or at least adding another sb1k. How big is the room? What is the exact tweeter height of the mains?