if say I have the verse roughed out on one project, the chorus on a second and the middle eight, at another tempo, on a third. How can these be built into a song?
Arranger can do it, but only if everything is in the same project.
The three projects would have different instrumentation.
In the legendary Sequential Circuits Studio 440 you could create a song from several sequences which kept their original time signature, tempo and instrumentation. It was very simple. Mind you both your helpful suggestions could do this with the help of Arranger. It would just be a bit messier at first.