This is definitely a good move by Ainge. I wouldn't get overly excited. As others have pointed out, Thornton is a Jordan Crawford type, Zeller's ceiling is likely as a decent backup center, and the first rounder is likely going to be late. None of that's a game-changer. Still, considering we essentially gave up nothing to acquire some assets, I'm definitely on board.
I wouldn't worry about having too many guards. Johnson, Babb, and Pressey all could be cut and it certainly wouldn't be the end of the world if Smart and even more so Young got eased into the rotation.
And with only three big men worth playing (Sully, Olynyk, and Zeller) and only two SF worth anything (Green and Wallace), I wouldn't be surprised to see us trot out some small ball lineups with three guards and/or Green/Wallace at the 4.