'Dancing With The Stars' -- Fashion Rules for New Cast

Famously eccentric designer Betsey Johnson is bringing her brand of crazy to "Dancing with Stars" ... along with the much younger YouTube fashionista Bethany Mota.

As we told you ... ABC had been looking to sign a big social media star ... and found her in 18-year-old Mota ... who's amassed more than 7 million fans with her YouTube tutorials on fashion, hair and makeup.

Sources close to the show tell us ... producers have also tapped "Back to the Future" actress Lea Thompson, Jonathan Bennett from "Mean Girls," Janel Parrish from "Pretty Little Liars," and Sadie Robertson -- granddaughter of "Duck Dynasty" star Phil Robertson.