Just from the top 25 guards in the nfl right now:
Brandon Fusco, G, Vikings, played at Slippery Rock as a Tackle, then switched to center. Played 3 games at guard his rookie year(2011), and then all 16 games the following year and 15 last year.
Todd Herremans, G, Eagles, played at Saginaw Valley, out of 48 games, he played 40 at right and left tackle in college. Although he started only 4 games his rookie year at OT(2005), he was then switched to guard in 2006.
(already mentioned) Jahri Evans, G, Saints, played at Bloomsburg, played all of college at tackle, drafted and started all 16 games and two playoff games his rookie year. and was named to the Pro Football Weekly All-Rookie Team.
Again, that was from a top 25 list I found online. Something else I came across, is that most all of the top 25 guards in the league, played tackle in college, whether it be at a division I or small school.
Good post man you took the research further then I did