I came up with a way to have the Young Six exist in my AU, cause I like them.
Short version:
It takes place in my a hypothetical version of season 10, based on the comics.
Twilight is still the ruler of Equestria. Many different creatures are starting to live alongside ponies and each other. The other creatures grew their own Trees of Harmony, and each Tree is connected. Unbeknownst to them, the connected Trees of Harmony have created their own hidden Tree.
There are still some minor conflicts involving the different creatures living together, but that's to be expected.
After the discovery that all creatures can use magic with enough practice in season 9, different creatures now attend Celestia's School for Gifted Unicorns... um, Creatures.
One day, a rhino comes to Equestria. He's one of the few creatures not happy about these changes, so he had gone on a quest to figure out how to destroy each Tree.
Long story short, after discovering the new connected Tree of Harmony, the Young Six use its Elements to defeat the rhino. Twilight and her friends see something in these children, and decide to become their mentors.
A pony named Luster Dawn, who was already Twilight's pupil
A yak named Yona, Applejack's pupil
A dragon named Smolder, Rarity's pupil
A griffon named Gallus, Rainbow Dash's pupil
A kirin named Sandbar, Fluttershy's pupil
A hippogriff named Silverstream, Pinkie Pie's pupil
Bray the Donkey (who was reformed in the last season) is still Spike's assistant, and occasionally joins them on adventurers. Later on the rhino gets reformed and becomes Starlight's pupil, and joins the Young Six.
The rhino is this mysterious villain from one of the a future murals:
Is it just me... or does it look like they're in outer space? My Little Pony in Space 🤣