A fixture at second base with the Seminoles, Thomas made his professional debut on Wednesday in the Arizona Rookie League with the Mesa Cubs.
"We're very happy to get him and I think he has a chance to be a real good hitter," Cubs Scouting Director Tim Wilken said of Thomas.
"He has some things to work on defensively, but once he gets with (Cubs Minor League Infield Coordinator) Bobby Dickerson, he'll smooth up even more. He's got plenty of arm at second, runs the bases well and has some pretty good speed," Wilken added.
Barney, also 21, was the Cubs' fourth-round pick from Oregon State. He was a member of the Beavers' back-to-back College World Series championship clubs and batted .302 in 67 games this past season.
"He's a very good player," Wilken said of Barney. "He was a catalyst on their championship club (in 2006) and can handle the bat. He's a winner. Our area guys were really strong on him."
Barney also made his professional debut on Wednesday in Arizona.
Unofficially, the Cubs have signed 29 of their 50 overall selections in the 2007 First-Year Player Draft.