The 6-foot-4, 318-pound Guy spent part of last season with the Colts but was waived in August during the final cuts.
Nixon, a 6-6, 314-pound tackle, had been on Washington's practice squad.
To make room for the players, Indianapolis (2-1) waived receiver Da'Rick Rogers and running back Kerwynn Williams.
Rogers joined the practice squad on Sept. 2 and was promoted to the active roster on Sept. 19 when Indy was hit by a rash of injuries.
Williams, a seventh-round pick, was on the practice squad until he was promoted to the active roster when Vick Ballard went down with a season-ending knee injury two weeks ago.
Online: AP NFL website: www.pro32.ap.org
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