As long as Kelly doesn't have to defend some of the quicker power forwards in the NBA it should be good. Horford should deal with the tougher defensive assignments, he is the better defender.
I think on offense both can stretch the floor pretty well. There will be times when one will have to work from the paint. The beauty of this is Kelly has play making skills Amir doesn't have. He rebounds just as well too. When KO is behind the 3 point line, he demands coverage, the floor will really open up for IT, and Smart, and AB to go to the rack.
All hopefully in theory.