Allen was previously a graduate assistant in Bloomington before his father's firing.
Hardy spent five years in Happy Valley.
Herron verbally committed to Penn State on December 11.
Andrew Cooke and Jonathan Evans join the Nittany Lions to fight for the No. 1 spot following Kris Shakes' departure.
Thomas is the seventh player to commit to Penn State's recruiting Class of 2025.
Mercedes-Benz Stadium has a capacity of 71,000, but there will likely be more in the stadium on gameday.