i have a theory that the School of Gifted Unicorns is a way that Celestia finds a unicorn that has the capability to rule Equestria. (ended up being Twilight) Along with this unicorns have and can only perform magical capabilities that go along with their talents(stated in season 1.) So i believe most very powerful unicorns since they have either suns,stars,or moons as their cutie marks would be able to raise the sun and moon as alicorns but it will take a lot of training. Candace would not be able too do this since her talent is spreading love and the moon and sun have nothing to do with that. So i believe when twilight becomes older (an of age alicorn but that’s another theory i have for another day, which includes the moving magical mane) she will be able to raise the sun and moon. And she is at this age in the last episode of My Little Pony