Smith played under Matt Campbell at Toledo from 2012 to 2013.
Taupaki is the second former UCLA defensive linemen to follow Ikaika Malloe to Happy Valley.
Davis recorded 13 total tackles this season, with his best game coming against Clemson, when he tallied six tackles in Boston College's loss.
Penn State turned the ball over 25 times in the loss.
Gilliam will rejoin Jim Knowles and Anthony Poindexter Jr. with the Volunteers.
Schlaffer left the program midseason and was the first to enter the transfer portal.