Washington was previously committed to Alabama since January 2022.
Stokes flipped his commitment to Florida less than a month after the Nittany Lions lost an offensive lineman to Georgia.
USC and UCLA will become the Big Ten's 15th and 16th teams beginning in 2024.
Mack will join high school teammate Conrad Hussey in Happy Valley, who committed to the Nittany Lions' Class of 2023 in April.
In-state kicker/punter Ryan Barker announced his verbal commitment to the Nittany Lions' Class of 2023 Tuesday night.
Nelson is the tenth recruit ranked as a four-star or higher in the Nittany Lions' Class of 2023.