Can I use a non-standard design for a stairwell ceiling cutout in an old house?
Unfortunately, as with most questions like this, you're going to have to talk to an engineer. A bunch of amateur DIYers on the internet, is not the proper resource for this type of information.  

You're making a major structural change to a building, and you don't want it to cause property damage or personal injury. The "correct" and responsible thing to do, is to seek the advice of a trained professional.