As far as I know, modular stairs can be adjusted slightly by dragging the bottom footprint. Place the stair, but don't release the mouse. This allows you to move the bottom of the footprint, and make some adjustment to the stairs. I don't know if this still works in OfB, as I haven't yet built a new lot, but it has worked prior, so I suspect it still will.
I have my first floor built - entire lot is flat, and there are no floor tiles on the first floor. I put in my three squares of foundation, and then the one square of stage. When I go up to the second floor though, it won't let me place floor tiles where I need to in order to connect the stairs to the stage. And if I place the tiles beforehand, it won't let me build the foundation!!