Celestia is Rose, obviously. And since Rose is Pink Diamind, the same applies to Celestia. Ponies in this AU are kind of amorphous and purely magical beings, so they can change their form as they see fit, or combine it through fusing. As Rose, Celestia has her lovely multicolored hair and lack of wings or horn to hide her alicornhood. While Pink Diamond form is a younger looking pink maned alicorn.
I mostly chose Twilight and Tia for this due to Rose and Pearl's relationship seems to share a few qualities. Their adoration for their loved one, their need to clean and tidy, and highly intelligent.
Tia and Twilight fuse into Cadance! It seems a little weird, but I find Cadance's design to be a near-perfect fusion of the two. Their similar/related color schemes is what really sold me on it.
Also, Greg is Shining Armor! Not a 100% sure if he'll stay Shining Armor, still debating between him and Flash.