Scott, like Harris, is a former Tennessee Titan. Scott played in Tennessee from 2008-11, meaning he was there for Jim Schwartz's final season with the Titans. Scott also played for the Indianapolis Colts from 2004-07 and the Philadelphia Eagles in 2012.
At all three stops, Scott was a starter. He's made 128 career starts over the course of his career, and he's actually only been active for three games where he didn't start (all in his rookie season).
With the Lions, Scott could potentially start at right guard, but Larry Warford remains the favorite to win that job. He certainly has more competition with the addition of Harris on Monday and Scott on Tuesday, but the expectation is for these guys to ultimately be competing for backup roles when training camp and the preseason come to an end.