If I bake the cupcakes (about 40) and then (while still warm) cluster them together in the shape of the "cake", about how long after that would I want to wait for them to cool so I can frost/decorate them?

Thanks!

Just bake and let them cool then put them together into your shape for the "cake". Cupcakes don't take that long to cool completely.

Gotcha. I read online somewhere, that when making a cupcake cake, it's best to use warm cupcakes when putting together the design as when they are warm, they are easier to push together.........

I've never heard that. You'aren't pushing them together. You place them on your board and line them up the best you can so that they fill the gaps. It's a tearaway cake so the people eating it will still want a "normal" looking cupcake.