Bogut would be a fine backup.
I wouldn't start him and Horford together. Too slow on defense and would get in each other's way on offense. Both guys like to play off the elbow and are at their best when used to facilitate the rest of the offense from there.
An Amir Johnson (non-guaranteed contract) for Bogut trade makes sense.
I think they'll be fine. I think Horford and Bogut can play a lot of in and out together. And opponents will have a tough time getting into the paint with those two.
I think the offense will be a lot like when D-Lee and Bogut played together. Only without the shooting of Steph Curry (Isaiah), Klay Thompson (Avery) and Harrison Barnes (Crowder).
I think the team loses more offensively than it gains defensively by starting the two of them together.
Bogut has always been at his best when he has had a quick footed mobile PF next to him. Draymond Green in GSW. Mbah a Moute in Milwaukee. Guys who can move around the court and take advantage of his passing ability. Which is much harder to do when the paint is clogged. Like it was with D-lee and Bogut and often would be with Horford and Bogut too.
I think they can play together some of the time but I wouldn't start them together.