To clear up something, the Penguins are NOT over the salary cap with this signing.
https://www.hockeyscap.com/teams/penguins
Prior to this move, hockeyscap listed them with a full 23-man roster: 13 forwards, 8 defensemen, 2 goaltenders. At that point, the Penguins had $472,500 of cap space.
To add Cullen, one must remove a person from the 23-man roster. Do that by swapping out Bryan Rust ($652,500 for Cullen ($800,000). It's a reduction of $147,500 in cap space.
It'd be nice to move a Scuderi or Kunitz to shore up the defense, but a move is not necessary to get under the cap. The Penguins are under it no matter what combination is used to get to 23 players: Sundqvist up, Sheary up, Wilson up, Erixon or someone else as the 8th D. Doesn't matter. They'd be under the cap no matter what.