How to tell when I have the right ribbing?
You are asking all the right questions, and thinking about the right variables, so that's a great start.  There a few other things you might consider as well:

Rib knits stretch more than stockinette, but stockinette will stretch.  Acrylic and wool yarns are usually far stretchier than cotton or linen.  A 4x4 rib is stretchier than a 2X2 (for example). In addition to these stretch variables, as you have pointed out, the degree of heaviness of your base skirt will also contribute to the final garment stretch.   

Even testing each independent variable (e.g., the waistband rib) one at a time, in isolation of the system, could result in a misleading result.  For your example of knitting a section of waistband with various ribbing, at a minimum you would need to knit each test band the actual length you plan to use since the stretch of the band will change with increased length.  But you will still not really know how the band will hold up under the weight of the base skirt. The rib band might need to be knitted on smaller needles to provide more stability and less stretch.  You don’t say what skirt length you plan to make, but obviously, the shorter the skirt the less pull it will have on the ribbed waistband.

I also think the successful execution of your design depends your body type, e.g., if you are curvy with a small waist you will need to cast off stitches at the waist when you start your rib if you work from hem to waist, or cast on if you work from rib waist to hem.  

One idea that might help with your questions specific to the waistband would be to hem the top edge and run a drawstring or wide elastic through it so you can tighten it if needed.

Without making a prototype skirt and adjusting it to meet your needs, similar to making a muslin in sewing, there's no surefire way to calculate the simultaneous effects of all your variables on the final garment. 

I suggest looking at knitted skirt patterns to get a general idea of what your initial yarn, needle, rib, etc. variables should be and adjust from there.  For example:

https://www.pinterest.com/pin/235805730472519799/

Your project sounds like a great idea, good luck!